ANSWERS:
(Add your own in comments)
BDSM is a technique to allow you to achieve reproductive
success. In animals, normally a dominant partner is the one to spread his or
her genes, so it makes sense to be attracted to dominance as you are mating
with the head of the group.
In contrast, if you are a low ranking member, one of the few
ways you may be able to become attractive to someone else is to make them feel
powerful and in control.
BDSM is an extreme manifestation of those urges.