Say all you want about the Angry Birds and Green Pig's rivalry, but this image of the Angry Birds and Green Pigs celebrating christmas together (Even having the King Pig dressing up as Santa Claus and happily giving out presents to the birds and pigs) is an unusual yet very wonderful sight to behold! ◊
The video for the second Christmas themed expansion. We could go over all the tropes as to why it's Heartwarming, but it's better if you see for yourself.
As strange as it may seem, this anti-bullying video of the Black Bird defending a little pig from Three Evil Mexican Pigs is a sweet sight to behold, even though the green pigs are naturally the enemies of the angry birds. 
At the end of the Angry Birds Trilogy Trailer, we see the CGI-Animated White, Blue, Yellow, and Red Birds laughing and having a great time while sitting on a couch, like any happy family would! Isn't that adorable or what? 
The end of the Angry Birds Toons episode "Another Birthday": After spending his birthday feeling dejected and unwanted, the green pig with the birthday hat returns to the pig pen with the others at night, the lights go out, and then the piggies wish the birthday pig a happy birthday!
The end of "Cordon Bleugh", where the pigs discover that White's cooking (which the other birds had turned down, finding it inedible and causing her to cry) is utterly delicious. The smile on White's face is the cherry on the sundae!
"The Portrait" from the Stella episodes features Willow forced to paint a portrait of Gale. During this time, she also helps out an artist pig who's making his own portrait. After Gale shows disgust for the pig's portrait and breaks the pig's paintbrush (leaving the poor guy in tears), Willow hands the pig her own paintbrush. The pig happily responds by giving her a small cuddle.