Pop culture reduces It's a Wonderful Life to that last half hour, and thinks the whole thing is about this guy traveling to an alternate universe where he doesn't exist and a little girl saying, "Every time a bell rings, an angel gets its wings." A hokey, sugary fantasy. A light and fluffy story fit for Hallmark movies.
But this reading completely glosses over the fact that George Bailey is actively suicidal. He's not just standing there moping about, "My friends don't like me," like some characters do in shows that try to adapt this conceit to other settings. George's life has been destroyed. He's bankrupt and facing prison. The lifetime of struggle we've been watching for the last two hours has accomplished nothing but this crushing defeat, and he honestly believes that the best thing he can do is kill himself because he's worth more dead than alive. He would have thrown himself from a bridge had an actual angel from heaven not intervened at the last possible moment.
That's dark. The banker villain that pop culture reduces to a cartoon purposely drove a man to the brink of suicide, which only a miracle pulled him back from. And then George Bailey goes even deeper into despair. He not only believes that his future's not worth living, but that his past wasn't worth living. He thinks that every suffering he endured, every piece of good that he tried to do was not only pointless, but actively harmful, and he and the world would be better off if he had never existed at all.
This is the context that leads to the famed alternate universe of a million pastiches, and it's absolutely vital to understanding the world that George finds. It's there to specifically show him that his despondent views about his effect on the universe are wrong. His bum ear kept him from serving his country in the war--but the act that gave him that injury was what allowed his brother to grow up to become a war hero. His fight against Potter's domination of the town felt like useless tiny battles in a war that could never be won--but it turns out that even the act of fighting was enough to save the town from falling into hopeless slavery. He thought that if it weren't for him, his wife would have married Sam Wainwright and had a life of ease and luxury as a millionaire's wife, instead of suffering a painful life of penny-pinching with him. Finding out that she'd have been a spinster isn't, "Ha ha, she'd have been pathetic without you." It's showing him that she never loved Wainwright enough to marry him, and that George's existence didn't stop her from having a happier life, but saved her from having a sadder one. Everywhere he turns, he finds out that his existence wasn't a mistake, that his struggles and sufferings did accomplish something, that his painful existence wasn't a tragedy but a gift to the people around him.
Only when he realizes this does he get to come back home in wild joy over the gift of his existence. The scenes of hope and joy and love only exist because of the two hours of struggle and despair that came before. Even Zuzu's saccharine line about bells and angel wings exists, not as a sugary proverb, but as a climax to Clarence's story--showing that even George's despair had good effect, and that his newfound thankfulness for life causes not only earthly, but heavenly joy.
If this movie has light and hope, it's not because it exists in some fantasy world where everything is sunshine and rainbows, but because it fights tooth and nail to scrape every bit of hope it can from our all too dark and painful world. The light here exists, not because it ignores the dark, but because the dark makes light more precious and meaningful. The light exists in defiance of the dark, the hope in defiance of despair, and there is nothing saccharine about that. It's just about as realistic as it gets.
