Humans are able to experience Orgasms at will. We deny this to function and to keep propagating the Species, but in fact the mechanisms are easily triggered if you know how In fact sexual stimulation simply results in us accepting that we are "allowed" to reward ourselves. Sometimes this denial fails in some people, but we ignore them and try to explain their ability with a disorder called Permanent Sexual Arousal Syndrome. Even though those people tell us that they simply have orgasms like we move our arms we ignore that and tell ourselves they have a hypersensitivity and still need some stimulation.
Sex and Orgasms feel good. They just reinforce some neurons. The neurons they reinforce dictate the behaviour of seeking out more of them, we rationalize with all of our strenght that there's a reason we display this behaviour.