Hypersexuality! Another article from this same journal discusses this topic. You can find the link here. Interestingly they also see this “disorder” as an extreme on a continuum. But in this case the talk about sexual behavior and preoccupation.
I, for one, am preoccupied with masturbation. I'm all the time masturbating for hours at a time over porn. If I was somehow a womanizer (a smooth talking stud with a huge cock) and had real women to perform all sorts of sex acts with I probably wouldn't masturbate so much, But that's not about to happen. The truth of the matter is that I'm full of lust and I keep it to myself. So I express my hypersexuality with masturbation. This is just how things worked out for me.
On the other hand I'm sure there are people who really don't feel much of an urge to have sex or masturbate. Then there are those in between who like sex and/or masturbation but aren't preoccupied with it. So I believe that people vary in their sexual drive (or whatever you want to call it) and I happen to be at one extreme. Guys also differ in how bold they are with women and I happen to be rather low on this dimension. But the two together (along with some other traits) and you have someone who masturbates every chance he gets.