You can help out a small town called Overlook in the Highlands by doing a few mission for them. The town is under Hyperion control, and is subject to constant abuse at the hands of Handsome Jack. (One such quote over the intercom states "Congratulations! The town of [Overlook] has qualified for double its medical shipments. Enjoy your [second] bottle of asprin!") Almost everyone in town has been afflicted by a degenerative brain disease known as Skull Shivers because of Handsome Jack's mining operations. The missions involve collecting Skull Shiver medicine for the town, restoring their medical machines and finally helping them get a shield like Sanctuary’s so they can be safe from moonshot barrages and finally be free from Handsome Jack’s control. The woman giving the missions, Karima, is one of the sweetest, most polite people on Pandora you’ll ever meet. Even dying of a brain disease, she still makes you give the medicine to the other members of the town (including a sexist dirtbag named Dave) before she takes any herself. Though that doesn't prevent her from arranging for you to blow up Dave's house with the mortar later on before she activates the shield. But, you know, he deserved it.
Also, see this article in which Anthony Burch talks about how Karima's character made a real person with a stutter feel, since characters with stutters are rarely so well-portrayed.
Tiny Tina telling the bandits that her "big brudder" is gonna go kick their asses is too damn cute.
Look around Tiny Tina's home; and you'll see a picture of her with Roland and she's hugging him.
One of the random things Tiny Tina does in her workshop is fall asleep on her bed and snore, with her legs sticking straight out. She'll eventually wake up with an adorable yawning sound that even the subtitles just call "(cute yawning sound)".
Fail the mission to bait Flesh-Stick by killing him (he used to offer a surprising amount of EXP, before the game was patched to keep people from just powerleveling on him), Tina comments that she told you not to kill him..."but it's okay, I still love you."
She gives you a quest to get an autograph from her third favorite mass murderer in the Campaign Of Carnage DLC. The guy says no, and you can tell how hard she's trying to hold back the tears, and she has you kill him and collect his head instead. Then after you turn the quest in she mentions her number one favorite mass murderer is you.
There is an NPC in Sanctuary that randomly spawns named Michael Mamaril. Talk to him and he'll hand you a piece of loot of blue or better quality. Michael is named after a Borderlands fan who died of cancer at the age of 22. Gearbox deliberately designed his NPC in such a way that players will want to search him out and ask for his help, ensuring that his name will live on in the hearts and minds of everyone who plays the sequel.
He's also outright stated to be a Vault Hunter, which in the Borderlands universe is synonymous with "total badass."
There's one in a mission objective, no less. After Handsome Jack has Angel sabotage Sanctuary's shields and Lilith is forced to phase the city to prevent it from being destroyed, the mission objective is "Search for your friends." You've only known these people a short time, but they've come to respect you, and to rely on you - and they're your friends.
When you collect Tannis' journal logs from the Highlands (which tell the story of how she adjusted to life in Sanctuary) it marks pretty much the only time in the entire series where she ends up happier at the end of them. Particularly when you stop and realise how desperate she was for friendship during the events of Borderlands ("Are we friends now?") and realise that the Vault hunters from the first game decided that, yes, they were friends!
One of the logs even mentions that she accidentally spits on Roland and he overlooks it because he knows what a hard time she's having adjusting to being around people again.
Every fan who played as Mordecai will call Bloodwing's death one of the saddest moments of the game. They're probably right, and it's heartwarming like you wouldn't believe to find out that Bloodwing laid eggs, one of which hatches during the credits.
To a lesser extent, the fact that Mordecai doesn't blame you for Bloodwing's death, despite you shooting her down. The possibility you did anything but try and help isn't even brought up.
As you gather the ECHO recordings in the Caustic Caverns, it is revealed that the crystalisks were docile and friendly creatures, and that Blue was even playing games with Booth, the Dahl mining group's security chief. Then when Dahl ordered them to be hunted for their valuable crystals, Booth refused, and the Dahl mining chief had her killed. This, coupled with the Dahl order to hunt them, turned the (apparently relatively intelligent) crystalisks hostile. You'll find Booth's grave among the Guardian Ruins, but in order to get there you end up fighting your way past more than a dozen crystalisks to get there. In other words, the crystalisks were guarding her grave, apparently aware that she was trying to protect them.
It's a tiny thing, very easy to overlook, but crops up in a couple of ECHO logs: it's nice to know that somewhere down the road between the present day and the day when the universe became a Crapsack World ruled entirely by megacorps valuing profit far above human life, gay marriage became such a commonly-accepted thing that even Handsome Jack doesn't consider it strange or worthy of scorn.
While its also a majorTear Jerker, hearing just how much Roland meant to the people of Sanctuary during the mission where you have to spread the word about his death. Even Marcus and Tannis are affected by it. Another heartwarming thing about the quest is, when you approach Marcus, you can find an ECHO log detailing Roland's Rousing Speech to the Crimson Raiders.
"Say not in grief, "He is no more." but live in thankfulness that he was."
After you save Lilith from the Bloodshot bandits, if you look at her surveillance screens, one of them is not surveying anything, but instead on what presumably might be Facebook for the ECHO Net. What is heartwarming about this is that she was posting on Roland's wall 3 consecutive times and each post is adorable.
Lilith: Was just thinking about you :)
Lilith: Was thinking about you, again.
Lilith: Hey, I love you. Call me.
Easy to miss when you're too busy shooting things to listen, but every once in a while one of the bandits will berate you for killing their companions because they actually liked that guy, or because they were actually friends with them. When most of their comments indicate that the bandits more or less just co-exist with each other, it's kind of heartwarming to know that some of them get along at least.
Roland and Brick coming to terms with each other after a misunderstanding that made the latter leave the group. The way Roland humbly apologizes and how Brick accepts it shows that although they had several disagreements they still are loyal and more than willingly to work together.
Gaige's dad has at least one in each ECHO broadcast Gaige does after the first. In the second one, he brings her hot chocolate. In the third, he's mentioned to have freaked out when she cut herself before getting her arm, but very clearly accepts her new one. To top that off, when she mentions that her rival stole her idea, he asks her, "You're going to take that bitch down, right?" Finally, he buys her time to get off planet, knowing he may never see her again.
You have to admit: It felt really good giving T.K.'s niece the plans for his Wave Gun as evidence that Hyperion stole it for their own gain, and knowing the evil company could lose millions in a lawsuit. It's nice to know the old man's death wasn't pointless.
In Lynchwood, the player characters can find a friendly Skag pup named Dukino chained up by the side of the railway. Scooter then gives them multiple missions to free Dukino, give him medicine, feed him until he grows into an adult, find him a nice big cave for him to live in, and to kill Dukino's mother, as Skags eat their young. Not only does the act of rescuing the little guy feel good, you also get to witness him running around happily and rolling on his back waiting for belly-rubs.
Towards the end, before the Vault Hunters go to take down Jack for good, they have the option to pay all the important named NPCs in Sanctuary a visit, so that they may give them some inspiring parting words and items (such as weapons, shields, etc.) to aid them in the final battle.
During these visits, the Vault Hunters overhear an ECHO conversation between Scooter and Ellie. Ellie has a request for Scooter: to tell Moxxi she loves her.
Orphan: So ends the story of how the Vault Hunters defeated the treacherous Captain Scarlett, and slew the Leviathan. And as the sun set on Pandora, the Vault Hunters realized that while they'd found Captain Blade's lost armory, the true treasure of the sands... had been inside them all along. The end. How was that?
Marcus: That last part was REALLY hokey.
Orphan: Yeah, if felt kinda contrived, didn't it?
Marcus: Extremely. But otherwise... you ended that story pretty well. For a dirty little orphan.
Marcus: Eh, don't mention it. I've gotta teach somebody to tell these stories when I'm gone. Might as well be you.
Orphan: Yeah. I love you Marcus!
Marcus: ...That's weird.
The Loader AI core from "Out of Body Experience". After the player puts him in three bodies, all of which he tried to kill you with, he decides to let you turn him into a gun or a shield, and help you kill people. Both items are quite useful, but the robot still talks to you as either one, and even refers to you as "Boss". It's actually quite enjoyable, having a non-human, funny companion playing the game with you, especially compared to the othertwo talking guns.
Shade is a surprising gleam of honesty in the Crapsack World of Pandora. He's clearly insane and will creep out players in the beginning of the Captain Scarlett DLC with his frantic desire to be "friends with you", and you'll most likely be happy to be rid of him and quickly enamored with Scarlett's Affably Evil cheerfulness. However, when she inevitably betrays you by sending the a small army after you, the first thing that happens after is Shade quickly calling you up with clear worry that you've been attacked, tells you where Captain Scarlett has run off to, and suggests that for your own sake, you settle things by pumping her with bullets. In a world full of dangerous lunatics and clearly distrustful characters, Shade honestly considers you his friend and is truly concerned for you.
Mr. Torgue, the epitome of Testosterone Poisoning himself, saying "Also, you should treat Moxxi nice! NOTHING IS MORE BADASS THAN TREATING A WOMAN WITH RESPECT!"
"Claptrap's Birthday Bash" is usually pretty sad, with Claptrap waiting for the guests that he doesn't know refused to come along, but it can become a player-made Crowning Moment of Heartwarming if all four players are there and stick around for the "party". You four have had to put up with Claptrap's stupidity and a level in jerkass he took ever since the beginning of the game - and yet, when he needs someone to spend time with, you're there for him. Even better, he happily says "THAT WAS THE BEST BIRTHDAY EVER!!!" at the end of the mission. Granted, slight Tear Jerker for how that's most likely true, but more Heartwarming that you managed to care for him where everybody else hasn't.
Overlaps with Crowning Moment of Funny, but this line from him during "The Talon of God" mission. He just sounds so damn proud.
Brick shouting "Stay the hell away from my friend!" when you're attacked in "The Talon of God". He had formerly shown you massive respect, true, but as a Slab. This is the first time you're acknowledged as a friend.
Some of Krieg's quotes when reviving others or being revived can be this.
Somebody still cares!?
I am unused to the emotions I am currently feeling!
Hearing gratitude and kindness out of Krieg is strangely endearing. And also, most of the time, a Crowning Moment of Funny. Especially that "Love you..." which is said as creepily as possible.
During Talon of the Gods Mordecai and Brick show up for assists until Mordecai and the barge is downed. Mordecai was willing to go down giving you and Brick cover fire but Brick would have none of that and immediately leaps in to save him. Unfortunately Jack decided to open fire on the ship with orbital weapons, and you quickly realize this means you'll be alone for the rest of the mission.
After the loss of Bloodwing, you can hear Brick relating to Mordecai the story of how Jack killed Brick's puppy. Brick, being himself, messes it up completely by going off on tangents. Fortunately Mordecai realizes Brick's intentions and appreciates the thought.
While it may also be sad, the good half of Krieg asserting some level of control over his usual self and reason to become a Vault Hunter? It was for Maya.
In particular, the moment where Krieg's psycho side, guilted by the inner voice, manages to maintain just enough coherence to warn Maya about the rats. Everything but one of his eyes might be covered in Krieg's psycho mask, but the look in that eye and his groan of pain says a lot.
At the beginning, when Krieg is about to attack someone:
Good!Krieg: I wanna warn him. I wanna tell him to run, to hide, to get out of sight so that I wouldn't have to kill him. That's... what I wanna say. What actually comes out of my mouth is- Krieg: (charges at the man) I HAVE THE SHINIEST MEAT BICYCLE! (leaps at the man, swinging his buzz axe) Man: (screams) Good!Krieg: (resigned) Heh... Close enough.
At the end, after Krieg saves Maya and she saves him in return:
Good!Krieg: Now tell her "thank you". Tell her that because of her, we might actually one day be able to act like a normal person again. (Beat) Krieg: (threateningly) I POWDERED MY COCKATIEL FOR THE RIBCAGE SLAUGHTER! (Another Beat, then Maya smiles at him.) Good!Krieg: (understandably surprised and delighted at the same time) Heh... Close enough.
And his sole coherent line in the film, in the correct context:
Turn around Pretty Lady!
The entire premise of Tiny Tina's Assault On Dragon's Keep: Lilith, Mordecai, and Brick, sitting down with Tiny Tina to play a game with her.
Tiny Tina hugging Roland's memorial statue at the end of the DLC, along with every single Vault Hunter, up to and including Krieg, present to pay their respects. Even Claptrap was there.
"Tina there are three full grown people here who just searched for imaginary crumpets because they thought it'd be fun. There's no such thing as adulthood".
At the climax, a crying, frustrated, and deeply upset Tiny Tina is on the verge of having the Handsome Sorcerer kill Roland in-game because that's what happened in real life and she's becoming depressed at finally admitting that he's really dead. The other Vault Hunters plead with her to actually "finish the story" in a good way and then she changes the ending: Bloodwing swoops in, deflects the spell off of Roland, and then kills the Handsome Sorcerer. The subsequent scene showing Roland with Bloodwing perched on his shoulder is equal parts awesome and awww.
Especially hearing Mordecai shout, "HELL YEAH!" when his old partner saves the life of one of his best friends. Doubly so since he didn't even want to play the game when they first started out.
Lilith defending Angel's memory when Tina forces the latter into a villainous role. Even after she very nearly got them all killed, the Vault Hunters are still able to acknowledge that Angel really was a good person in the end.
How about the fact that ALL of the Vault Hunters were at Roland's memorial, including the second generation. These are a group of people that go from mildly sociopathic to straight up insane, yet still respected Roland enough to pay their respects to him.
Relatively minor compared to everything else above, but when Lilith finally allows Mister Torgue to be a part of their game. He sounds absolutely ecstatic.
The fact that Tiny Tina has invited Krieg to her game of Bunkers and Badasses too.
The entire plot of the game kinda qualifies, especially if you've played the first game: In the first Borderlands, all of the vault hunters were generally pretty solitary and never really interacted much with each other. Altogether they come across as rather lonely individuals. Seeing them all working together and enjoying each other's company in the second game is rather sweet, if only because it shows that despite all the crap that they went through in the first game, they ended up being friends. Of a sort.