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Tear Jerker: The Fresh Prince Of Bel Air
In "Papa's Got A Brand New Excuse", Will's dead-beat dad, Lou, comes back into Will's life after fourteen years, and Will tries his best to try to get to know him better. Even after Uncle Phil calls out Lou over not being there for him when he was a child, Will — for the first time in the show — outright defies Phil by telling him that he isn't his father. In the end, Lou's true colors show as he becomes more concerned over some new business and has to cancel a planned summer trip with Will. It's clear from their last talk together that Lou's not coming back — and Will doesn't want him to come back. Will tries to shake it off and defiantly yells about all of the things he learned to do — and hopes to accomplish — without his father around to help him. He eventually breaks down as the weight of the situation fully hits him. Will finally just looks at Uncle Phil with the saddest look on his face and says one of the shortest, yet saddest lines ever uttered: "How come he don't want me, man?". Phil then gives him a Cooldown Hug. The camera then zooms in on the present Will had bought for his father: a wood carving of a father holding a young son. Link.
What's also sad is that there's a still of that statue as the credits roll. It gives us enough time to see that the statue says "#1 Dad".
Also look carefully at the hug; Will's hat gets knocked off during the hug, symbolizing that he's a scared, sad little boy still asking "How come he don't want me?"
IN ADDITION: His tears were for the friends that he knew growing up that didn't know their fathers.
In "Bullets Over Bel Air", Will takes a bullet for Carlton when the two are mugged while withdrawing money from an ATM machine. After seeing Will hurt so badly, Carlton's feelings of helplessness push him over the edge, and he ends up buying a gun. Will ends up begging Carlton to get rid of it so that he doesn't intentionally or accidentally hurt someone in the same way Will was. After convincing Carlton to give up the gun, Will breaks down in tears. Link
And, to make it even sadder, Will checks if the gun was indeed loaded, and then when he did check, he realized that, yes, Carlton actually bought a gun and loaded it with ammo with the intent to shoot someone out of self-defense, or to defend a loved one, if the need came for it. This is Carlton we're talking about.
Will confessing that the pep pills that almost killed Carlton were his. A true He Really Can Act moment by Will Smith as he slowly breaks down during his speech explaining the whole situation. Link.
After Jazz inadvertently throws Phil's chances at being a judge into jeopardy, Will blows up at him and tells him to leave.
Will: So, as of now, this friendship is over, you're not welcome here, so I'd like it if you just got the hell outta my crib!
Most of the series finale, for long-time viewers of the show. Even Carlton's So Bad, It's Good dancing to Tom Jones, because it was the last time we'd ever see it... and even more so when it was Will's idea and he actually joined in! In this case, though, you'll most likely be laughing your head off and tearing up at the same time.
"Home is Where the Heart Attack Is"
Will tearfully admitting that he "Caused" Uncle Phil's heart attack by giving him a cheeseburger he bribed him to buy.
Carlton being afraid to see his father in such a vulnerable position, saying that he's like Superman to him. This one is softened by Mood Whiplash, though it still works pretty well in the context (one can see it as Phil not wanting to see his son so sad about his state):
Phil: And cheesecake is my Kryptonite, huh?
This is also the first time Will ever talked about his own worthless father and uses that to push Carlton into seeing Phil.
Carlton: I just don't want to see him with tubes hanging out.
Will: There's gonna come a time when you do see him with tubes hanging out.