evolution - Why do we grow so much hair on our heads compared to our bodies?

I've been wondering about head hair, facial hair in particular. Human males can grow very extensive beards should they choose to not shave - however you do not really see this in our chimpanzee cousins! Yes, they have little pseudo-beards, but the difference being that they do not shave, that is just the length they reach. Whereas in humans we can grow to our hearts content (*this may not be the case, see this question).

I can't really see why this would have been selected, unless it's simply that (evolutionarily speaking) women like men with long beards?

So my question is: why can humans perpetually grow head hair, yet we have lost the majority of our body hair, in comparison to chimpanzees and other ape family members?