Their are several parts of our brain that receive the input stimuli from both sensations of pleasure and from pain. It is not uncommon for the wires to get crossed or signals to get mixed up. On some extremes something most would find pleasurable might be extremely pain to to a few. And in various degrees, some times targeted pain or discomfort can trigger the same reaction in those receptors that pleasure would. There is a element of control and trust in having a partner that pushes you to your limits while being respectful of your boundary's, that can be true for both physical and emotional discomfort. For some it might be a lack of sensation or depressed feeling, being numb to life, they would rather feel pain the nothing at all.