And then when he's reborn as Jack Frost, he gets all giddy over his new powers - only to reach a human settlement and find out that no one can see, hear or touch him. Damn that hurts.
When you realize that Jack's little sister WATCHED HIM DROWN.
Jack scoffing at gift-giving as bribery makes more sense when a flashback reveals Jack's past life and family were obviously poor and he would have rarely gotten them himself.
The Darkest Hour, where Pitch breaks Jack's staff and leaves him and Baby Tooth at the bottom of a crevice, while Sandy is dead, children have stopped believing in Bunny, and lights all over the world are going out.
How broken hearted and crushed Bunny looks when he finds out the children stopped believing in him.
Bunny: They don't see me...they don't see me...
That entire scene, from the moment Jack realizes what's happened on, is just heartbreaking. And like a lot of things in the movie, it's the little details that make it the most powerful, like that fact that it's not Bunnymund who gives Jack his What the Hell, Hero? moment. No, it's North. North, who's been nothing but supportive of Jack the entire movie, to the point of being a mentor, almost a father-figure. Between how disappointed and betrayed he sounds and Jack trying to figure out what he can say to make things right, plus Bunny losing his cool for the once and only time in the movie, and Tooth essentially in mourning...it gets you right in the gut.
Even worse, when Bunny yells at Jack, he looks as if he's going to hit him!
Bunny shouting that they shouldn't have trusted Jack is made even more heart-wrenching by the fact that in their last scene together, the two had finally started to warm up to each other. When the others objected to Jack being the one to take Sophie home because Pitch was still at large, it was Bunny who said that they needed Jack with them.
He may be the embodiment of fear, but you can't help feel kinda sad for Pitch for his despair over not being believed in.
Even sadder, if you've read the books then you know that Pitch had a daughter, and was tricked into opening a Pandora's box prison because he was a Papa Wolf and wanted to protect her.
Note in the Antarctica scene, right before his We Can Rule Together spiel to Jack, how angry he sounds when he responds, "NO?! I don't know what it's like to be cast out?!" when Jack screams, "YOU DON'T UNDERSTAND ANYTHING!" after Pitch told him that he understood how Jack felt. Then there's how sad he looks and sounds when he says, "To not be believed in?! To long for...a family?"
The family part is even sadder after you read the spoiler above.
In the same scene noted above, Pitch's face looks genuinely sad for a beat when Jack refuses his offer.
Most likely because the offer was almost definitely genuine, and not just some attempt at manipulating Jack.
When Jamie runs through him at the end, Pitch looks distraught at not being believed in again. What tops it all off is that the Guardians look... genuinely sorry for him. Seriously, the guy just needs a hug.
Towards the beginning, Jack asks the moon if there's anything he's doing wrong and why people don't see him. Then he walks along the powerlines, tapping his staff against the wires to freeze them. The music playing during the scene and the visuals cut such a lonely image you really have to feel sorry for the poor guy.
And he's speaking in such a broken voice which doesn't help matters at all.
Watch Jack's face every time someone makes a comment about his invisible status. Notably during his and Bunnymund's argument at the Pole and when Pitch insults him at the Tooth Palace. He's unable to shoot back a comeback and he looks like he's about to cry each time.
Tooth's expression when she realizes all the little kids will wake up with their teeth still under their pillow.
Pitch harassing Jack in his domain and tempting him with the memories of his past, while Jack begs him to stop.
The last bit in the whole meeting feels like twisting the knife when you learn how bad it's gotten.
Jack: You don't know what I am!
Pitch: 'Course I do, you're Jack Frost! You make a mess wherever you go, why, you're doing it right now.
Jack: ...What did you do?
Pitch: More to the point, Jack, what did you do? *Cue evil laugh*
The opening scene when Jack realizes no one can see him. It's not really that sad the first time you watch it (more like shocking) but then you watch it again and it hits you that this is probably the village where his family lives. They're probably grieving for him, unaware he's alive and well and RIGHT THERE near the village, only they can't see him and he doesn't remember them.
At least we get one Tear Jerker that brings tears of joy. The scene in Jamie's bedroom when he starts to believe in Jack is very sweet and genuine. The poor kid's been invisible for 300 years, you can see the emotions he's going through when he says "He sees me!'' EVERY SINGLE TIME.
The bit before the credits. "In dedication to Mary Kathrine Joyce". Mary Kathrine is William Joyce's (the original writer of the books) daughter; it was in fact her who first gave Joyce the idea for "The Guardians", and all of this - the books, the movie - was his gift to her. She died from a brain tumor at 18 - before even the first book was released.
A prequel comic made by one of the movie's creators reveals that Pitch was actually part of one child's best memories. It was a boy with a cleft palate living in the middle ages, who was rejected for his minor deformity. Pitch was the only one who would talk to him as if he were a normal person, albeit Pitch makes it clear that he found amusement in waiting to see him get rejected. In the comic, Tooth stares at the boy's baby tooth and wonders if telling Pitch about how important he was to the boy could have reformed him.