Why is it always "Us Against Them"?

I've been in the workforce long enough to witness countless "us against them" situations. Whenever there is a problem, it is always "It's not our fault", "It's not our problem", "It's theirs, let them work it out" and countless other variations of it.

Why? Apparently, it's because we're wired like that. Read about the BusinessPundit's introduction to David Berreby's book "Us and Them: Understanding Your Tribal Mind" here