Why is David being invited?

W: I really can’ t stand the way David controls the conversation all the time. If he’ s going to be at the Christmas party, I just won’ t come.
M: I’ m sorry you feel that way, but my mother insists that he come.

W: Prof. Clark said that the mid-term would cover the first fifteen lessons.
M: Really? I thought it only included the first twelve lessons. Then I must spend the weekend to go over the rest of the lessons.