I've definitely got a sadistic streak, and I can safely say I've thought about it a fair bit without coming to any definite conclusions. It is worth mentioning that I have a masochistic side as well (though I don't like to be dominated, per se, I sometimes have my sub "top" according to my directions), and so some of my interest in inflicting pain is, I think, the result of my own personal enjoyment at receiving it. But I hardly think that accounts for the whole mentality.

Since you're a psych major, you are certainly familiar with Freud (whether you have read his actual works or not; unfortunately many psych programs skip source material for interpretation), and while he has certainly developed a reputation far beyond the scope of his works, I think his understanding of the subconscious is vital to any understanding of sadism. For me, I can clearly recognize that my own sadistic tendencies were not, for a long time, conscious. Whatever their original cause - whether it be experiential or genetic or something else, I'm not sure - it wasn't until a certain point that I accepted that they were there at all. Many psychologists consider things like masochism and sadism to be clinical pathologies, but I couldn't disagree more, if only because the conscious recognition of a pathology - and the acceptance of it - is meant to be one of the chief means of overcoming that pathology. For me, however, I openly embrace my sadistic and masochistic tendencies, and I am highly aroused, at times, by inflicting pain on my submissive. Far from "curing me," my conscious recognition of my sadism has refined it in particular ways (I'm partial to nipple torture), and has allowed me to interact with my darker desires in a more healthy way.

I think that's kind of the key. I would guess that a lot of people who are cruel and demeaning in day-to-day life are actually repressing their own sadistic (or at least dominant) tendencies. In person I'm about as easy-going as they come, and I would never dream of hurting anyone, but before I recognized my sexual sadism, I would sometimes tend to fly off the handle and lash out at people when frustrated, or, worse, I would be terribly passive-aggressive, undercutting the self-esteem of others and trying to hurt them (an obvious outlet for sadism). The same energy that makes me sexually sadistic was, by being repressed, working to make me cruel and subversive with others, which in turn only fueled my sadism. Again, I have no idea why that part of my subconscious comes from, but it is certainly better for me to acknowledge it is there.

As for what makes me tick, when it comes down to it, I suspect it's a little bit sadism, a little bit projection, but mostly its two other things: my sub's reactions, and control. There is perhaps no more immediate way to get a reaction out of someone than physical pain. Many of us, at even the slightest prick, will yell "ow" even though the thing that pricked us didn't actually hurt or draw blood or anything. Combine the immediacy of pain-reactions with the mere fact of control - that is, the fact that a submissive is willing to undergo pain simply because you wish her to - and you have an intense combination. When inflicting pain, you can see and hear submission in an unadulterated way, because, while someone may serve you, may strip for you, may have sex with you, and may do any other number of things, only someone who has submitted to you (whether for an hour or for a lifetime) will willingly allow herself to by hurt by you (unless, of course, that person is a pain-slut, but many masochists do not usually inflict pain upon themselves). So, in that sense, I think what really makes me sadistic is the proof of submission that it offers.

Also, on a related note, it is for this reason that I am in awe of (and terribly attracted to) submissives.