I don't know but I have a theory that some people might have "the brain" for porn. It's like some people are more prone to become addicts than others, at least I have seen it happening. Me and my 1 year older cousin used to watch porn at the same time and even together, we used to do the same things porn related until he stopped and I'm here. I suspect that my fantasy obsessed brain had something to do with it. It wasn't much until porn that I watched started to create fantasies in my mind. I don't know, it could be the brain changes, the memory of pleasure or whatever that make people come back to it even after a long time or maybe just the fact that their brain has that "thing" for porn. It's just my guess and also curiosity.