This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Monsters University
The constant teasing and belittling Mike goes through by being told he will never be scary or fit in with his peers. Kids, jocks, Sulley, and even Dean Hardscrabble tell Mike that he wasn't meant to become a scarer.
It gets even worse when Mike has to find out the hard way that they were all right. It gets even worse when you consider that, arguably, they're not right. But it didn't matter in the end. All that mattered was the moment when Mike believed them.
After Mike and Sulley are expelled, they share a touching moment with each other and Dean Hardscrabble before parting ways. The BGM playing when Mike was on the bus really tugged on heartstrings because he had just lost his lifelong dream.
"You said you believed in me." The fact that Sulley lied when he said it hurt Mike a lot more than finding out about the cheating itself.
You know what makes that moment worse?Mikey was doing an homage to the Scarer he saw when he was little, the one who inspired him to become a Scarer in the first place. When he performed the winning scare, thinking he was a full scarer and then knowing that Sulley rigged the testing device was salt in the wound.
And this line
Sulley:What was I supposed to do? Let the whole team down because you don't have it!?
Mike: Don't say that, please don't say that. You do NOT know how I feel.
And then there's how guilty and upset Sulley himself sounds when trying to get through to him, his arrogant façade now broken, and finally admitting to why he acted that way.
Sulley: I act scary, but most of the time... I'm terrified.
Mike:How come you never told me this before?
Sulley: Because we weren't friends before.
Oozma Kappa when they find out Sulley cheated in the final event of the Scare Games. They don't turn on Sulley or even say anything, they just leave the unearned trophy on the ground and very sadly and silently slink away.
The fleeting look on Mike's face when he finds his prized hat damaged after escaping the human world. The audience knows that this represents Mike's loss of a lifetime dream.
Dean Hardscrabble confronting Sulley over whether or not he thinks Mike is scary. He tries dodging it a few times and argues how Mike's hard working and very intelligent, but the question is repeated twice before he sadly falls silent, because they both know the answer.
The picture of Sulley shanking hands with Waternoose can be considered this to those who remember how the friendship between them ends in Monsters, Inc..
The fact that Mike not only fails to achieve his dream of becoming a scarer — he fails to ever be recognized, celebrated, or even respected in his career.
Randall betraying Mike to join ROR. Even worse is Randall is totally unsympathetic to Mike's current plight.
I'm finally with the cool kids Mike. Don't blow this for me.
Making this worse is the Fridge Horror - Randall is socially awkward and shy.