Persons who are abused often internalize the idea that they are deserving of abuse. This simple and fairly irrational idea can easily survive into adulthood and find expression in bondage and dominance activities. As a child one is not capable of rationally examining and rejecting the idea that one is deserving of abuse. The patients put themselves into dangerous situations that are similar to past traumatic situations in an attempt to master the frightening situation. Another way to think about BDSM activity is that it involves very intense stimulation.
People can become 'addicted' or attracted to such intense stimulation.
You may also appreciate the intensity and drama of a self-loathing or 'slutty' feeling that might come from recognizing that you 'like' BDSM practices even while knowing that 'nice' girls don't like that sort of thing. There are many people out there who self-harm by cutting or burning themselves, often in an attempt to recover from feelings of dissociation, to punish the self for being 'bad', etc. Your motivation for allowing others to harm you may or may not be similar. I'm also reminded by your letter of battered women who 'dance' with their husbands in cycles of abuse. It may be that you like the making up period that comes after being degraded.