This post made me think about the reasons for sadistic or masochistic interests (as one hardly works without the other). Warning: I'm not a psychologist, so I have not studied psychology and am purely acting on my practical knowledge of the human mind.
The basic question for most people seems to be: why would somebody want to injure someone else (and, even more pressing a question, why would somebody want to be injured by someone else)?
The basic question for me is: why should they not?
I might be a bit biased on this, as I was mobbed in school and thus have seen the sadism that resides within teenagers, but as far as I see it, both sadism and masochism - in a weak form - are embedded in the human nature. But why am I thinking this?
Both the will to lead and to be led are part of human nature, too. If we didn't have the will to lead, there wouldn't be anybody around who could tell others what to do. If we didn't have the will to be led, nobody would listen to the leader. (And what about SM? We'll come back to that later, don't worry.)
Now, "leading" means taking control of a situation. That's what a sadist does: taking over complete control, including the control over the well-being, possible pain and even possible death of others.
And "being led" is nothing else than giving over control of a situation to someone else. That's what a masochist does - and that's why there has to be a lot of trust between sadist and masochist in such a relationship. (If you don't understand that statement, try to combine this paragraph and the one above.)
Humans like being in control - well, most of them. It makes us feel powerful and important to be asked for guidance - or even orders - by others. It makes us feel good. Having control equals having power over others. That's the sadist-part in us, I should say. Of course, that doesn't necessarily mean we all would like to really hurt someone. But the idea alone of being able to ... it's interesting and a little tempting.
But on the other hand, always being in control is also very tiring. People depend on you, they do not act or work without you supervising them. In business life, the working life - and thus security - of hundreds or thousands of people might depend on the decisions of one person. That is a heavy burden to shoulder. Giving over the control of your life to somebody else, can be very relieving, therefore.
For somebody who has been controlled by others - even mistreated or even abused by a person more powerful, being in control must be a great thing. It will make them more secure in a relationship to be the one in control, instead of being under control. That, I think, is why people with such a past tend towards being sadists. There might also be some revenge mixed in.
And somebody who thinks they've made great mistakes in their past, might see a masochistic position (including pain and humiliation) as some sort of atonement. But that's just a theory I have come up with.
Especially reading Bitchy Jones' blog has helped a lot with those insights, apart from my own looks into the darkness of the human soul (my own and that of others).
On the whole, I wonder more about people wondering about sexual or other preferences than over the preferences themselves. As long as it's consent between both partners, I don't see anything pathological or otherwise wrong with something two adults to with - and to - each other.