MARY: Is this the ear you can't hear on? George Bailey, I'll love you till the day I die.
Mary's childhood infatuation with George morphs into something with real staying power. Mary is the one who senses that they're meant to be together, and she has to take the lead. George eventually gets the idea.
GEORGE: What is it you want, Mary? What do you want? You want the moon? Just say the word, and I'll throw a lasso around it and pull it down. Hey. That's a pretty good idea. I'll give you the moon, Mary.
MARY: I'll take it. Then what?
GEORGE: Well, then you could swallow it, and it'd all dissolve, see? And the moonbeams'd shoot out of your fingers and your toes and the ends of your hair. ... Am I talking too much?
This is purely a lyrical romantic reverie. George wants to give her "the moon," something special and amazing. Mary doesn't really need that; she's happy with her dreams of an ordinary home and family.
GEORGE: Yes. Sam's crazy about Mary.
MA BAILEY: Well, she's not crazy about him.
GEORGE: Well, how do you know? Did she discuss it with you?
MA BAILEY: No.
GEORGE: Well, then, how do you know?
MA BAILEY: Well, I've got eyes, haven't I? Why, she lights up like a firefly whenever you're around.
GEORGE: Oh ...
George is so preoccupied with his own ambitions and dreams that he needs his mom to clue him into the love that's waiting right in front of him. Mary has never been distracted from her dream of loving George.
GEORGE: Now, you listen to me! I don't want any plastics, and I don't want any ground floors, and I don't want to get married, ever, to anyone! You understand that? I want to do what I want to do. And you're ... and you're ... [he suddenly starts kissing Mary] Oh, Mary, Mary.
Love trumps George's ambitions. He's willing to give up everything for Mary. Mary has had to watch him slowly come around to realizing what she's known all along. Up until this point in the film, the romantic story has had some screwball comedy elements. From here on, though, it's a story about steady, deep love and commitment.
MARY: Remember the night we broke the windows in this old house? This is what I wished for.
Mary was playing the long game; George was worth waiting for.
GEORGE: Mary, it's George! Don't you know me? What's happened to us?
MARY: I don't know you! Let me go!
GEORGE: Mary, please! Oh, don't do this to me. Please, Mary, help me. Where's our kids? I need you, Mary! Help me, Mary!
George lost sight of his love when he was lost in all of the anxieties about the Building and Loan. But, being momentarily deprived of it makes him see how precious it is.