Why are we fighting the buggers?

From the text:

"I've heard all kinds of reasons," said Graff. "Because they have and overcrowded system and they've got to colonize. Because they can't stand the thought of other intelligent life in the universe. Because they don't think we are intelligent life. Because they have some kind of weird religion. Because they watched our old video broadcasts and decided we were hopelessly violent. All kinds of reasons."