I think a lot of people are into S&M for 3 reasons (my own theories)
1. The pure power trip & sense of power - or giving up of power- is attractive to some people at a sensual level as well as within their personality.
2. There is a roleplay and some "acting" involved, which can be fun and edifying. Like a Method actor, the real pain or torment ads that much more reality to the roleplay.
3. Like most people, and like many ticklers, S&M people are at the core, sensualists. They like to explore the sense of touch (and the other senses as well). However, there is something within them that makes them take that exploration further, with pain & discomfort being nothing more than further exploration of the sense of touch - and the exploration of trust that is taken to an extent not even found in many marriages! I think deep down, it's about touch and the senses, pain is just one more "angle". Add to the fact that, like with tickling, an intense session creates a natural opium-like high from the increased breathing & heart rate, the influx of adrenaline, the changing levels of seratonin in the brain and the release of your endorphins which (with tickling) have no place to go.... so you just become relaxed and laid back when it's all over. I explained this theory to a woman who was very dominant and into light to moderate pain (and tickling if it drove the person crazy) and she agreed that it was a pretty logical assesment, so who knows....