The "rivers of pleasure", as the bunches of nerves in our brain that induce orgasms when stimulated are called sometimes, are evolutionary developments. The purpose of a species is to procreate and make certain the continuation of its kind.

To do that, they must have sex (duh). Of course, as all animals are lazy, they must have some motive, some incentive to have as much sex as possible. In comes the orgasm. According to the law of positive reinforcement, "If it feels good, we will do it again. If it doesn't feel good, we won't do it again." Orgasms feel very good. Let's fuck like rabbits (so it goes, do it like they do on the discovery channel).

The development of the presence of nerves that link to orgasms was probably slow and done through evolution. The presence of the orgasm was made for us to fuck as much as possible so the human race does not die out.

There was a psychological experiment done on rats. Scientists figured out where to poke an electrode into the brain to trigger orgasm in a rat. They hooked up the apparatus to a button in a cage, which was also stocked with food and water. The rats ignored the food and water, instead pressing the little red button as many as 60 times a minute to continue the stream of pleasure. They starved to death.

This could possibly be an explanation for the behavior of promiscuous individuals (aka sluts and man-whores).