Lumping together masochism and submission to some extent:

5. Thrill-seeking, i.e., the endorphin rush that follows pain. I suspect there are some biophysical reasons that people have different pain thresholds, and different abilities to enjoy endorphins.

6. Desire to be the centre of somebody's focused attention.

7. Eagerness to please a partner.

8. Many SM acts have a lower chance of disease transmission or of emotional attachment than regular sex. So SM can be used as a way of having casual sexual contact without the emotional or infectious risks of sleeping around.

When people go to horror movies or get onto roller coasters, it's because they find it thrilling to experience feelings/sensations that would be very dangerous and undesirable in real life. I think SM is much the same thing... a safe and temporary way to explore intense feelings.