JOSEPH: George saved his brother's life that day. But, he caught a bad cold, which infected his left ear. Cost him his hearing in that ear. It was weeks before he could return to his after-school job at old man Gower's drugstore.
This incident sets up George's devotion to his family right from the start of the film. It foreshadows later events that show us the interconnectedness of people's lives.
GEORGE: Why do you call this a happy family? Why do we have to have all these kids?
Is this George Bailey speaking here? What's happening? In the depth of his despair, George rails against the people he loves most. He's displacing his anger at himself for not providing financial stability for them. They feel like a burden to him right now.
PETE: Is Daddy in trouble?
JANIE: Shall I pray for him?
MARY: Yes, Janie, pray very hard.
This explains the prayers we heard at the very beginning of the movie. The love his family and friends have for George moves the heavens, literally. Who would pray for Mr. Potter? Hint: nobody.
CLARENCE: They're not there, either.
CLARENCE: Zuzu's petals.
Since George has never been born, the petals from his daughter's flower vanish from his pocket. It's a symbol of all of the family love that would never have existed without George being around to create it.
MA BAILEY: When'd you see him last?
GEORGE: Today, over at the house.
MA BAILEY: That's a lie. He's been in the insane asylum ever since he lost his business. And, if you ask me, that's where you belong.
Without George around to keep Uncle Billy on track, he falls apart. Billy is one of those eccentric uncles who makes his way in life because he has a strong family to support him.
CLARENCE: Your brother, Harry Bailey, broke through the ice and was drowned at the age of 9.
GEORGE: That's a lie! Harry Bailey went to war! He got the Congressional Medal of Honor! He saved the lives of every man on that transport.
CLARENCE: Every man on that transport died! Harry wasn't there to save them because you weren't there to save Harry. You see, George, you've really had a wonderful life. Don't you see what a mistake it would be to throw it away?
Here's the callback to that first incident in the film. Clarence is moving in for the big finish. Since George loves his family more than anything, Clarence knows how to convince him.