I dont understand whats so funny. Please explain.

Me: Thanks Adam.
Adam: For what?
Me: Reminding me to start my car.
Adam: Its no big deal.
Me: To me it is. That would be a long, cold drive home. And I saw you doing other nice stuff for people today. And I appreciate niceness. Greatly.
Adam: Like what did you see me do?
Me: I dont wanna say because then when you get to heaven you wont get credit for it.
-EVERYONE IN THE SHOP BURSTS OUT LAUGHING-

Whats so funny?

I dont get it.