All species of animals that reproduce sexually have an innate drive to reproduce in order to propagate the species. For humans, the drive to have sex is a complex series of interacting factors that go beyond the basic need to reproduce. While it has been shown, but not proven that a few animal species such as dolphins and apes engage in sexual activity for reasons other than reproduction, sex for the sake of sex is something that exclusively a human trait.
Motivation for sex in humans has to do with emotions, attraction, desire, hormones, smell, visual stimuli, physical touch, environmental factors, classical conditioning, cultural attitudes, and cognition. It has been debated that there exists a difference between the sexes in terms of what arouses and attracts. Also, what determines sexual orientation and the what motivates attraction to the different sexes, or to both?
Obviously attraction is what motivates all species to engage in copulation. In the animal kingdom it is often the biggest, strongest, most flashy, one that gets the mate. This courtship ritual plays out in different ways, dancing, flashy fur, or feathers, fighting, or singing. But, we humans have complicated the matter down to an individual preference that has little to do with survival and reproduction, as it does with pleasure.