People expect their friends to help with their big events.
Dave is on the phone with his friend Phil who invites Dave to a party the next day and asks him to come early to help set up.
Dave wants to respond to Phil's invitation.
Dave tells Phil he'll definitely come early and help Phil set up and stay for the party.
Dave tells Phil he'll come to the party, but he has some other plans beforehand, so he can't help set up.
Phil is delighted that Dave is willing to pitch in and that he's not a selfish friend.
Phil is annoyed that Dave only wants to attend the fun part of the evening and refuses to pitch in.