Sulley telling Mike he is scared of failure, and only because he now considers him a friend.
This last line from Dean Hardscrabble, where even though she keeps Mike and Sully expelled, wishes them good luck.
Dean Hardscrabble: Oh, and Mr. Wazowski? Keep surprising people.
And the gentle smile she gives at this scene. Remember, the Dean does not smile throughout the whole film, only at this scene.
"Perhaps I should keep an eye out for more surprises like you." Because of their actions, Mike and Sully actually changed the University for the better.
The scene where Oozma Kappa spy on Monsters Inc. and Mike and Sullivan try to settle their differences. It's really heartwarming to see.
Also, the scene after this when Sulley comes out of his room wearing the Oozma Kappa sweater.
Mike and Sully's Group Hug with the Oozma Kappa fraternity Club near the end of the film. No words need to be said to describe that scene because it's just that heartwarming.
The Oozma Kappa fraternity Club Group Hug was fully drawn and coloured, as well as included in the children's illustration book of Monsters University.
Mike and Sully's happiness at doing their jobs as they rise up the ranks of Monsters Inc. From Mail-room operators to Scare-team in the first film!
The montage where Mike and Sully finally start to get into a groove, cooperating and working together, eventually waking up at 6 AM and getting ready in a symbiotic, almost choreographed routine. It just shows how well they've come to know and work with each other.
Everything about Randall, before he decides to join ROR. Mike cheering him up and assuring him that his invisibility is cool. The scenes with him quizzing Mike for the tests, and just how happy he is to be helping.
Despite never becoming a scarer himself, Mike still gets enough recognition for his later work in Monsters Inc. that he gets included in the Scare Cards, which have the only good, full image photo of his. Being a collector himself, and considering how happy Mike gets when his partial image appears in any important photo, you can imagine how much it means to him to have his full image in a Scare Card, right next to his best friend.
Even better is when you realize that Mike would, in the end of Monsters Inc., have an even better career as a comedian to make children laugh. After all those years of staying backstage, Mike finally takes the spotlight at the Laugh Floor and does a great job doing it. Heck, he is such a success that he even currently has his own hit comedy show at Walt Disney World!
The intro with Mikey as a child visiting MI on a school trip. Frank McCay, one of the Scarers, actually stops to share some nice words with the class. Mike then gets the idea of sneaking after Frank into a child's room while he does his job. He manages to be so still and quiet that Frank doesn't even notice him and is immediately impressed. So impressed, the he gives Mikey his MU baseball hat that Mikey cherishes for most of his life.
The smile on Mike's face when he departs Frank's room. He had just witnessed greatness and fallen in love with it. It sets off all his hard work and effort to attain this greatness.
Also, doubles as both a Freeze-Frame Bonus and a Call Back, but the way Mike scares the test dummy child in the last round of the Scare Games resembles how he saw Frank McCay back when he was little...erm...more little than he was, scare the child back at the beginning of the film.
Along the lines of the above, just before Mike performs his scare, he hears one of his classmates, Johnny Worthington, and Dean Hardscrabble telling him he doesn't belong. But the last thing he thinks of is Sulley coaching him the night before. Perhaps it's Foreshadowing their relationship as Heterosexual Life-Partners.
Terry and Terri seeing the three-headed monster at Monsters Inc. and realizing there can be a place for them as scarers. The looks on their faces really sold it.
Similarly, Don seeing an older monster working as a scarer and realizing that being older doesn't make him useless.
Don Carlton's farewell to Mike and Sully, asking them to keep in touch and handing them his usual business card. On the card, Don had scribbled the word "Scarer" over his original occupation title.
Don revealing he and Sherry are getting married.
Oozma Kappa going berserk when they discover that Mike is stuck in the dangerous human world. And even after the revelation of Sully's cheating, they still love and trust him unconditionally to help him rescue Mike.
OK's willingness to stand up to Hardscrabble when she shuts down the door to the human world while Mike and Sully are trapped inside. Ineffective, but it shows how much they care about their teammates.
Don: They're still in there!
Hardscrabble: Until the authorities arrive, this door stays off.
Squishy:[charging into the room] No! You can't do that!
Sulley's reaction. His charging through the door into the human world to rescue Mike somewhat resembles his rescuing Boo from Randall and Waternoose's using the Scream Extractor on her.
Mike rescuing Sulley when he can't get up the cliff.
A subtle moment at the ROR party. Squishy does his little dorky dance and coax Sully to join him at the dance floor. Sully just stands by with a frustrated sullen expression, since he still considers it a disgrace to be part of O.K. But after many gestures from Squishy, Sully relents, grins, and joins Squishy on the dance floor, showing that he is warming up to his O.K. brothers.
Sully finally expressing his respect and understanding of just what makes Mike special: "Mike, you're not scary. Not even a little. But you are fearless." Doubly heartwarming when you realize that Mike settling for "It's ok to be ok," seems to be the film's message, only for Sully to subvert this message and understand that it seems ok at first, but in truth, it's extraordinary.
Mike's farewell to O.K. "you guys are the scariest out there, and don't let anyone else tell you otherwise." Mike, as well as Sully, have to live with the dishonor of their expulsion, but Mike was able to give O.K. their happy ending.
For that matter, the whose sequence with Mike's locker, where you see that, working from all the way at the bottom, Mike and Sully managed to make their own happy endi- ah, beginning, actually!
The way Mike is breathing in the attention of a group of MU students in the "he's the heart and soul of the team" scene, as well as Oozma Kappa making it very clear that every single one of them recognizes him as their leader. After all the struggling and the put-downs, both must feel incredible to him.
Even when the Dean forcefully asks Sully if he thinks Mike is scary, the first two times he adamantly responds, "We couldn't have gotten this far without him!" and "He's the heart and soul of the team!"
He cares that much for Mike that he does some last minute training on practical scaring with him, telling him to dig deep. It becomes a Tear Jerker when he doesn't have the heart to tell him he doesn't have it and he considers cheating to spare Mike from the truth.
It was foreshadowed previously Sully was willing to cheat in order to get himself in the Program. But by the time he becomes friends with Mike, Sully does cheat but with a more benevolent intentions for both Oozma Kappa and Mike.
"Stop being a Sullivan and start being you."
After young Mikey emerges from the child's bedroom into a crowd of frightened Monsters Inc. workers, a Freeze-Frame Bonus would show that Earl "The Terror" Thompson was among those who witnessed Mike's first taste of the Scaring Career. It resonates with even more meaning when years later, Mike spotted him at the factory again and gushed over him.
According to the collectable Scare Cards, Frank McCay and Mike share the exact same birthday (November 16). It's a small but sweet touch by the creators to draw another connection between the two.
Mike and Sulley's talk to each other by the lake. It may be a Tear Jerker, but when Sulley tells Mike that he feels he's an insult to his family's reputation, Mike asks him why he never told him that before, promoting him to reply that it was because they weren't friends before. It was that moment when Sulley fully accepted Mike as his friend.