One who is full of himself fills all the space around him. There is no room left for anyone else. Therefore, he despises another person by virtue of the space that other person consumes. He may give reasons for his disdain, but the reasons are secondary.
This is called wanton hatred. It is the reason given for our exile. It is the core of all evil.
Its only cure is wanton acts of kindness and caring for the other guy beyond reason.