This story is an aftermath of "A Canterlot Wedding," it goes into bigger detail on what Princess Celestia, Shining Armor, and the Mane Five are all quite surprised that Twilight Sparkle shows no resentment for leaving or shunning her. Even though they've shown regret for it, Twilight Sparkle insists not to be so hard on themselves, and that their actions are understandable.
Until later on... She admits that she felt resentful towards them for abandoning her but her friends assured they were going to talk to her after the reception, and never really going to abandon her.
And to show her that they care about her, they recount all the times that their friendships were tested, but ultimately led to happy memories.
On that same note, both Princess Celestia and Shining Armor insist that if Twilight had been wrong about the fake Cadance, they would have forgiven her.
Equally heartwarming are the lines that immediately follow. Applejack says that Twilight made them all worthwhile, a point which Twilight shoots down without missing a beat. What makes it heartwarming is what she follows up with:
Twilight Sparkle: Everything that I've ever done worth bragging about is because I've had one of you at my side, keeping me strong. It isn't me that makes all of you worthwhile...it's the other way around. I don't want you to ever forget that.
This gem during Luna and Celestia's conversation after the wedding.
Celestia: You know, I've always envied Twilight's parents. Because no matter how much Twilight might love me, adore me, and strive for my approval, they will always hold the place in her heart that I want.
Luna: You love her that much?
Celestia: Like a daughter.
Luna tried and failed to bring Celestia out of her guilt over how she ignored Twilight. The younger alicorn then suggests bring the older's student in so they can reconcile and talk it out. Celestia begs her not to, feeling that she doesn't even have the right to look at Twilight at the moment. Luna goes and gets Twilight anyway and lets her into Celestia's room. When they begin to apologize to each other for what happened at the rehearsal, Celestia asks her student if she can sleep with her, like how Twilight used to fall asleep at her side after a long day of work. Twilight smiles and admitted that she was hoping for the question to pop up.
Luna gets asked by Celestia, and later Applejack and Rarity, if she would have believed Twilight if she was at the rehearsal. Luna, while not sure, admits she probably would've at least checked into Twilight's claims, since Twilight willingly helped Luna on Nightmare Night and convinced Ponyville to give her a second chance after the whole Nightmare Moon fiasco.
Twilight's reaction to finding out that Spike wants her to be his mother. After initially being stunned, she's deeply touched by the idea.
The entire sequence between Twilight and Spike in the "Lullaby" chapter. Words do not do it justice.
After Twilight and Spike finally mend the rift between them, Scootaloo is still disgusted with how Spike tried to avoid his problems and how he was throwing his guilt out-of-proportion. While Rainbow Dash agrees with her student that Spike was acting stupid, the blue Pegasus says that since she wasn't really herself after the drama at the wedding, either, she really didn't blame him for his emotional breakdown.
After Twilight's third Nightmare Sequence, she accidentally wakes Spike up when she bolts upright in bed (he moved back into her room after he got over most of his guilt). Instead of complaining, he climbs into her bed and pulls her into a comforting hug akin to how Twilight would help Spike with his bad dreams when they were younger.
When one of Olive Branch's co-conspirators comes to Ponyville to defame Spike for drawing a gang of dragons near town as part of some tyrannical plot of Princess Celestia's, how do the citizens respond? By running the conspirator out of town for picking on one of their own and insulting their princess. Even the Keeper compliments the townsponies for their valor.
What makes it truly heartwarming is who the first pony is to interrupt the conspirator: Bonbon, the one pony whom Fanon has painted as the most critical of the Mane 6's (and Lyra's) usual shenanigans.
While Pinkie's history with her actual parents fall under Tear Jerker, her history with the Cakes is pure this. Mrs. Cake first met her as a hungry foal and offered her some baked goods. Pinkie decided to help and ended up spending the night. She never left and the Cakes basically became her parents.
Mrs. Cake: You know we've never regretted taking you in, right?
Pinkie: I know. I've never regretted coming here.
The Tear Jerker quality of Pinkie's family history is what makes her reunion and reconciliation with them all the sweeter.
Rainbow Dash letting Scootaloo sleep at her house after her parents are arrested.
Soarin' telling Rainbow Dash that he'll help her take care of Scootaloo when she adopts her.
It's a milder example, but it's touching that Owlocious has taken on a mentorship role for Peewee.
Rainbow Dash and Soarin supporting Scootaloo through the whole trip.
The way Scootaloo describes how everyone in Ponyville, even strangers of helping her in someway, and says that they're way better than her own parents.
By the end of the visit Scootaloo finally starts calling Rainbow Dash Mom and Soarin Dad, even if he and Rainbow aren't engaged yet.
Soarin promises that even if things don't work out between him and Rainbow he'll still be there for Scootaloo.
This is then followed by Rainbow joking they should make it official quickly since they're in Las Pegasus, since they are already raising a child, they practically live together, and already have "fun" with each other. Doubles as a Crowning Moment of Funny.
Rarity dropping everything, even her part in the investigation, to care for Sweetie Belle when she gets sick.
The exchanges between Rainbow Dash and Scootaloo showing that the latter is Happily Adopted.
This is especially poignant after the main characters fight Trixie and everypony finds out that the CMC explored the Everfree Forest at night. Rainbow sternly tells Scootaloo what she and her friends did was wrong, and punishes her by having her help out on Sweet Apple Acres while Applejack recovers from the injuries she got from Red Hoof. However, at the same time, Rainbow listens to Scootaloo about why she did it, and the older Pegasus never even thought about doing corporal punishment. Then to let Scootaloo know that everything is still fine between the two of them, Rainbow Dash leaps on her and tickles her as "another part of her punishment". All this is a far step up from what Scootaloo's biological parents were like.
Fluttershy trying to help a down Peewee.
Rarity taking care of Sweetie.
While trapped by the Changelings we see four of the five mane members' greatest dreams.
Applejack has a happy life on the farm with her parents still alive, Twilight coming by to help her, and the whole family about to celebrate Apple Bloom's cuteceañera. Also apparently Big Mac is dating Cheerliee.
Rainbow is captain of the Wonderbolts, has adopted Scootaloo as her daughter, and is married to Soarin.
Rarity is among the elite in Canterlot and engaged to Trenderhoof but has not forgotten her roots in Ponyville. Sweetie Belle is with her as well and is the youngest singer at the most exclusive club in Canterlot with Spike as her manager and coltfriend.
Fluttershy has reformed all of their old enemies to help them care for the animals around town, and has even tamed the Everfree Forest. To top it all off she has been accepted by the Canterlot Orphanage to come the next day and take home a foal.
Pinkie was not affected by the trap because as she said herself, she already has everything she wants in life after reconnecting with her old family in the previous story and still having her new one. She decided to just do what she can to make everyone happy.
The Changeling Queen tries to remind the Mane 5 about how badly she tricked them with her Cadance disguise during the Royal Wedding, in order to let their guilt get to them during the ensuing fight. Their response? They laugh their heads off and mock Chrysalis for thinking that was actually going to work since it's been an entire year. Sure, they still feel bad for not listening to Twilight at the rehearsal, but they've moved on from their guilt and aren't letting one bad mistake control their lives. That's some major Character Development for all of the Mane 5. It also counts as an Awesome moment.
Rainbow Dash being willing to give Gilda a second chance, since she genuinely misses their friendship.
Sweetie Belle forgiving Trixie for what she did to her while corrupted, inspired by Twilight forgiving her friends and family for the rehearsal incident, how Rarity prevented Trixie from being a victim of mob anger when she was arrested, and how Pinkie reconnected with her biological family.
Through the Mirror
Despite just meeting Twilight and Spike and knowing absolutely nothing about them, Principal Celestia offers to help them and takes them home with her so they can explain what's going on to her.
After Twilight explains everything to Celestia and Luna they offer her a place to stay until she can go home and decide to make a cover story for her, that she is their niece visiting from out of town.
Many students at CHS defended Rainbow and Fluttershy from other homophobic students who believed the rumor on how they were in a sexual relationship.
Flash helps Twilight with her research on how her the human versions of her friends broke up. Afterwards when he finds out she doesn't know how to dance he takes her to the school roof and gives her some quick lessons as her partner.
Even though she currently absolutely despises Rarity thanks to Sunset's manipulations, Rainbow is still adamant that Rarity didn't deserve what Sunset did to her.
In "Another Lesson", The Keeper meets with Twilight and teaches her not to assume the Humane 5 are going to be the same as her friends by showing her several different universes where various villains won and where Celestia and the Mane 6 are evil. The universe that horrifies her the most, though? A universe where Twilight refused to forgive her friends for what happened at the wedding rehearsal and bitterly ended her friendships with them.
Twilight: Please, just tell me I wasn't that cruel to Spike! Keeper: Of course YOU weren't. That Twilight wasn't you. That's the entire point of this!
Fluttershy genuinely misses her friends and despite being terrified of working with them and against Sunset, she's willing to help Twilight.