This explains why we have kids...

I guess it explains S&M too though....

From ScienceDaily
For years, the brain chemical dopamine has been thought of as the brain's "pleasure chemical," sending signals between brain cells in a way that rewards a person or animal for one activity or another. More recently, research has shown that certain drugs like cocaine and heroin amplify this effect -- an action that may lie at the heart of drug addiction.

Now, a new study from the University of Michigan adds a new twist to dopamine's fun-loving reputation: pain.

Using sophisticated brain-scanning and a carefully controlled way of inducing muscle pain, the researchers show that the brain's dopamine system is highly active while someone experiences pain -- and that this response varies between individuals in a way that relates directly to how the pain makes them feel. It's the first time that dopamine has been linked to pain response in humans.