Among the other chemical reasons people stated, I think part of the evolutionary fact is due to humans being able to mate any time of the year. Many animals have their instincts kick in at the prime time to mate, which coincides with the right time to give birth and raise offspring. Humans are adaptive and can rear young anyplace, any time. So instead of having our sex horomones go in overdrive in the spring and have us so horny that we rape anything we see, we got lucky and had it feel good, so we'd do it all the time. Although I have no idea if this is true, you could always go with the idea that somewhere there was a mutation that gave sex pleasure, and naturally those organisms would fuck all the time, have the most offspring, spread the bling bling gene, etc. But what do I know, I'm no biologist!
