Sex!
No, I’m not just trying to grab your attention! I stumbled across an article in the Wikipedia on the
evolution of sex. As in, why did we evolve sexual species (where you need two partners to reproduce) as opposed to asexual reproduction (where every creature is able to reproduce by themselves, effectively creating “clones”).
The point is that from a naive viewpoint, asexual reproduction is far easier - you don’t need to search out a partner, and your species can grow in numbers much more rapidly. However, sexual reproduction lets you evolve far easier to a changing environment, as well as distill out the best genes.
Of course, you then need to motivate the members of your species to actually have sex - which is presumably, and with all tact here, why it’s generally a pleasurable experience!