Really interesting thing about serotonin: in long term relationships (after the "honeymoon" period) brain scans have shown large dips in serotonin to levels akin to people with Obsessive Compulsive Disorder. Apparently this could be a evolutionary trait that resulted from thousands of years of monogamy in our society.

So if you get that unhealthy possessive feeling towards your SO, you're not crazy! I always found knowing the chemical processes that take place in our brain make relationships a lot easier to decipher. Takes a lot of the guessing out to just be like "Oh, I'm just on drugs."