When it comes right down to it, it all is brain chemistry. The question of why an individual has the specific brain chemistry is the one we don't always have the answer to.
A couple of years ago I was at a conference, specifically regarding addiction. They have discovered that a lot of things that were thought to be "decisional" were actually not decisional on the part of the person affected by addiction.
The person who is addicted thinks that they are making decisions, and the people around them think they are making decisions, and, under observation, it superficially looks like they are making decisions, but when you start isolating out their behaviors, you find that they are acting out behavior patterns that they are completely unaware of.
In the end, they do make a decision, such as to buy a bottle of wine, or methamphetamine, or something that they are addicted to, perhaps it is to cheat on their spouse with yet another person. However, they get to that final decision point with a lot of very subconscious behavior patterns that lead them to that decision point.
Part of the treatment of addiction, whether it be sexual, or otherwise, is to recognize these behavior patterns and interrupt them. This may be one of the reasons that going to Alcoholics Anonymous or Narcotics Anonymous helps some people. Changing behavior patterns is very hard. In the absence of self awareness, we tend to continue doing things that we have done before, regardless of the consequences. Frequently, that is because we don't actually recognize the consequences as being linked to our behavior patterns.
The science on this is still evolving.
My FWS was reading a book about adult children of alcoholics approximately 10 to 12 years ago, during early years of reconciliation. Her father is an alcoholic, it left her somewhat shellshocked. She said she felt like someone had followed her around and written a book about her life.
She wasn't even aware that she was repeating a cycle over, and over, and over, and had been doing so for long before she met me. After she met me, she still repeated the same cycle, but it was much slower being spread out over of a much longer period of time.