Not all the traits that are there became that way because of some – well as of yet unhidden reason (but that our storytelling minds most certainly can confabulate one for). Preferences can be arbitrary, and then take off. A bit like fashion sometimes, perhaps. And, this may be a bit hard to wrap ones head around.
Carl Zimmer has a great post up on this null-hypothesis (it is from 2010 – on the web time is not of the essence), with links to academic papers on this.