If OP is still reading this. This is the difference.
The first 20 or so times with someone are really awesome (with the first few times being out of this world). After that, if you have it too often (or always have it with the same person), it just becomes sexual release, and kind of devolves into a porn like experience.
IMO half of the fun of sex is the excitement of being so close with another human being. It's like nothing else you can do with another person. But eventually the boundaries between you and that person break down, and it's no longer nearly as fun to be so close to that person.
Women tend to not feel this way, but I think a lot of men have had a similar experience to this.