An odd friendship between Kent and Bunky. Kent's a homophobe, Bunky's openly gay. The two become best friends in the house, Kent even lets Bunky sleep inside the Head of Household room.
The twins joining up in Big Brother 5 US.
Dick attempting a Heroic Sacrifice to keep his daughter safe, by waking up everyone up at 2 am
In Big Brother 9, the houseguests had a food challenge where they had to make a gravy river to the food groups they wanted. They all actually put aside their differences and were working together for the common goal of getting beer and the right food groups - and this was one of the most argument prone houses in the game. The shot at the end where everyone had lined up smiling and admiring their handiwork made many viewers smile.
During one of the Veto competitions in Big Brother 13, every participant recieved a prize (ranging from the Power of Veto to non-beneficial items) and could swap their's with those of the previously eliminated contestants. Shelly (who was on the block) was eliminated first and received the POV, and Jordan was elminated second and got 24 hours of solitary confinement and a call from home. She swapped with Shelly, but not because the latter was on the block. It was soleyl because Shelly would get a call from home and be able to talk to her young daughter.
Big Brother 14 Jen and Danielle talking about Jenn's coming out. Heck, for that matter, homosexuality in the house has always been treated rather well - there have been gay people in the house but almost nobody's ever gone out on them for being homosexual.
Anyone who is more familiar with recent season may find it shocking to go back and see the big deal that was made about Bunky's coming out.
Donny in Big Brother 16 saving Jocasta from the block in Season 16 by using the Veto, stating that she should not have to face eviction as sick as she was.
Frankie sits in the Head of Household room by himself talking about his family and cancer.
Derrick intentionally throwing an HOH Competition and becoming a Have-Not for the week so that Frankie could get a picture of his grandfather who recently passed away from cancer.
Derrick is HOH, and he hears eliminated contestants saying "Holla, Derrick" and brushes them off... until he hears his daughter say "Holla, daddy!" and his face lights up.
In BB 16 Jeff and Jordan returned to the house and Jordan thought they were hosting a luxury competition what actually happened was that Jeff proposed to her in front of the world, her family comes rushing in, and they along with the current houstguests have a concert by Brett Eldredge while a montage plays of Jeff and Jordan's best moments in the Big Brother house. To seem them come full circle was touching to see the least.
Also from BB 16, Derrick, Donny and Nicole all helping Victoria when she faints in the bathroom.
Within the first two weeks, Audrey as a meta-example. When she reveals that she's transgender to everyone, a lot of players and fans alike took a very unique yet heartwarming response to it... Said nothing, just referred to Audrey as "She" and "Audrey". (This means a LOT) The Zingbot 3000 Twitter account said "Transphobic comments will not be tolerated here", and several websites literally put up instructions on what was transphobic and warned that they wouldn't be tolerated.
Another Audrey example. After Audrey has been put up on the block for her dishonesty, she enters a Heroic B.S.O.D. and isolates herself from the house, covering herself up and staying alone in the Diary Room and Have-Not room. James makes her a nice omelet for her.
Post game, but since the time Vanessa entered the house, same-sex marriage was finally made legal in the United States. In the post-game After Party, her girlfriend proposed to her.
Big Brother Aus:
Bradley, the only nerd in the house for the Season 9 revival so far, gets instantly loved by all the women in the house. They all even go "Awww" when they see him on screen.
While the real intention is to win a challenge for Surly, Zoe really opens to up to Sarah in bed about her fears of the second house (which they don't know is fake currently) as Sarah calms her down and gives her a good hug. Some time after that, Zoe gets her reward from Surly in the diary room. The dinner she missed out on last week after being forced to say yes to having Ray take her place in the captain's corners.
The show's treatment of Josh after he left the house because his brother had died. They stopped live updates, cancelled the current challenged and cancelled the sunday eviction to air a dedication episode to him. Along with that they donated the funds from the (now closed) voting lines to The Salvation Army which Josh and his family supported.
Likewise the housemates' goodbyes to Josh are very heartwarming.
The producer's have always been touchy about death close to housemates, like when they cancelled the Egg Sitting task after finding out one of the housemates had a miscarriage.
Delilah's "eviction" from the house in which she got a family chosen for her by the housemates. Upon leaving through the door, she got taken by a ninja. When the doors closed, the ninja was revealed to be the earlier evicted Stacey who had been with Delilah for most of her time in the house. The family chosen for Delilah The Bradleys were also there to take her.
Benjamin's win which hit its climax when he proposed to his boyfriend Ben (yes they have the same name) live. And Ben said yes!!!
Big Brother Canada:
When Andrew's twin brother came in to say hi to him.
From BBCan2, during Week 3 when Ika was up on the block against Heather, before the eviction vote, Ika put aside her Alpha Bitch tendencies (albeit until the check incident), built a pillow fort, and apologized to Heather for bullying her.
And even then, after the cheque Sadistic Choice, Ika delivered most of her Alpha Bitch attitude towards the first five - especially Kenny, Andrew, and Sabrina.
BBCan2: Kenny coming out to Sarah during the campout reward. It was very easy to see that Kenny lying about his being gay was really affecting him on an emotional level.
BBCan3 has Britnee and Sarah working together to win the HOH after the emotional eviction of Naeha, their closest friend and ally. After they won they hugged and whispered to eachother "For Nae. For Nae. For Nae."
Zach became somewhat infamous before his season started when it was discovered that during the first season of the Canada version he had made some very rude and nasty tweets to Peter about his gameplay, calling him a "bitch" and "pussy" with a "nothing life". When Peter finished the season and returned to twitter, he responded in kind with exactly the attitude he's now known for on the Side Show. Obviously, that's not the heartwarming part. That occurred when Zach was evicted and being interviewed by Peter on the Side Show. Peter brought up the tweets, geared up for the rant that everyone was expecting...and then forgave Zach. And then went on a monologue about how after Zach gets home he's going to get plenty of people talking trash about him online because it's easy, and that all that hate is going to wear him down, and that Peter will always be available to talk to if it gets to be too much for Zach since he's had to deal with the exact same thing.
Season 4 features Loveita and Kelsey's relationship changing over the course of a couple weeks where they go from being tense to each other to legit enjoying themselves in the secret suite.