Board Thread:Warhammer 40k General Discussion/@comment-24037808-20131031105702/@comment-3416287-20131031182718

The Chaos gods require the emotions from sapient creatures in the physical universe, but in order for sapients to survive there must be some kind of order, which we all know is the opposite of chaos. If the universe were to be consumed by chaos, sapients would die, which would deprive the Chaos gods of the energy they need to survive.

It's all about balance, if there is too much chaos, then the Chaos gods can't survive, but if you have some order, that provides sapients with the means to feed the Chaos gods. Chaos if unchecked will burn itself out.