It excites them to have power over others and they express that power sexually. While the victims of rape might not make a distinction between taken advantage of in a vulnerable moment, or being assaulted by a sexual sadist, I think there is an important difference. In one case, a person needs to learn not to use people for sex without their permission. I think most of these kinds of rapists can learn the difference between seduction and assault. Most of these people don't realize that what they are doing is wrong.
Rapists have an entirely different motivation.
They are motivated by the desire for power. First, I want to clear up another misperception. Well, to research the motivations of pedophiles I went in search of information about them. I found 209 links to pedophilic information using Webcrawler. Now, to the meat of the matter.