It is rude to refuse an expensive gift.
Charlie sees all his friends, including Dave, arrive at his birthday party with gifts.
Charlie wants to have the best possible time at his party.
Charlie realizes he's going to get a big gift from Dave, so he waits until everyone else leaves to open it.
Dave gives Charlie a Ferrari he won on a game show, Charlie refuses it because it might hurt the other's feelings.
Charlie opens the gift with only Dave there and thanks Dave.
Dave is offended and thinks Charlie must hate him, so he doesn't want to talk to Charlie anymore.