There are multiple theories for why sex is a strong motivation, and many fall under the theory of evolution. On an evolutionary level, the motivation for sex likely has to do with a species’ ability to reproduce. Species that reproduce more, survive and pass on their genes. Therefore, species have a sexual desire that leads to sexual intercourse as a means to create more offspring. Without this innate motivation, a species may determine that attaining intercourse is too costly in terms of effort, energy, and danger.