Why does Greg eat lunch in Mr. McCarthy's classroom in the novel, Me and Earl and the Dying Girl?

Greg is socially anxious and takes care to never shows allegiance to any particular group at school, so he can avoid having anybody dislike him. The cafeteria is the one location where this is impossible to achieve, because everyone is forced to make a decision about who to sit with for lunch, thus showing their allegiances. In order to get out of this problem, Greg usually eats his lunch in Mr. McCarthy’s classroom with Earl.