This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Monsters, Inc.
"Kitty has to go."
Imagine how Boo's parents must have been feeling the entire time Boo was gone. They could have spent the entire movie frantically searching for their lost daughter without a clue on where she is.
When Sulley accidentally scares Boo. The look on her little face is so heartbreaking... like her world just fell apart. And Sulley'sreaction, as he realizes through Boo's trauma just how he's been appearing to so many children in the past, and just what a terrible thing it is to be "Top Scarer".
Sulley: Did you see the way she... looked at me?
One of the most striking things about Sulley scaring Boo is how different her reaction is from what the scarers are trained to expect; the difference between her reaction and the robotic child's cartoonishly simplistic one perfectly shows how the monsters just don't see what they're really doing when they scare children. Also notable is that Boo doesn't actually scream; it's not just drowned out by by the robot child (the lights don't flicker like they do when she genuinely screams), she is too paralysed with shock and terror even to scream, and all she can do is cry at the betrayal.
If Boo crying after Sulley accidentally scared her wasn't enough, Randy Newman's score plays a sombre reprise of her leitmotif from the earlier scene of Sully trying to get her to sleep.
It becomes even worse after you watch Monsters University and you recall what Dean Hardscrabble told him about his scaring.
Just seeing how much the monsters traumatize children, how corrupt Monsters Inc. really is, and how the main characters in the prequel are unaware of that and strive to be a part of said company makes Monsters University a thoroughly depressing watch if you think about it for too long.
The shot-for-shot recreation of the Signature Scene from "Feed the Kitty", when Sully thinks Boo is being crushed by the trash compactor. Just like the original short, we laugh at the Black Comedy, cry at Sully's trauma and are relieved when the child/kitty are shown to be perfectly fine.
Sulley's Big "NO!" as he slams the door over and over again in anguish when he and Mike are banished.
Mike's "None of it matters" speech.
Mike: Sulley, what about everything we worked for? Does that matter, huh! And what about Celia? I am never...never going to see her again. Doesn't that matter? What about me? I'm your pal. I'm...I'm your best friend. Don't I matter?
Sulley: I'm sorry, Mike. I'm sorry we're stuck out here. I didn't mean for this to happen, but Boo's in trouble. I think there might be a way to save her if we can just get down to that—
Mike: We? Whoa-whoa-whoa, we? There's no "we" this time, pal. If-if-if you want to go out there and freeze to death, you be my guest. Because you're on your own.
Take a look at Mike's face after Sully leaves. He looks so guilty about turning his back on Sully, and also a mixture of hurt at being left behind.
This becomes even more painful if you've seen Monsters University and how hard and long it actually took for them to reach their dream of becoming scarers. Especially with the teasing and put downs by both students and teachers Mike had to suffer with in college because he wasn't good enough to be a scarer no matter how hard he worked at it. To Mike, Sulley was back to being that jerk back in college that cared about no one but himself.
Depending on your point of view Waternoose is this if you see him as a Jerkass Woobie. He was only driven to villainy out of desperation to keep Monsters, Inc. afloat in the midst of the energy crisis. It can be quite heartbreaking seeing him carried away by the CDA accusing Sullivan to be the cause of the energy crisis and that the Monsters Inc will be closed forever thanks to him.
Waternoose: I hope you're happy, Sullivan! You destroyed this company. Monsters, Inc. is dead! Where will everybody get their scream now!? The energy criris will only get worse because of you!
The worse thing is that when Sulley realizes that laughter is more powerful than scream and manages to both save Monsters, Inc and avert the energy crisis, Waternoose does get what he wanted, but unfortunately, he can't share in that victory.
When Sulley says goodbye to Boo. Sulley (at the time) knows he'll never see her again, but Boo's too young and innocent to truly realize what's going on. Once Sulley gives her one last hug and leaves, she runs up to the door and is ready to burst out to give him another 'scare' but all she sees is an empty closet.
Boo: Boo! (gasp)...Kitty?
Even Roz sounds like she doesn't want to have to do this.