Serotonin is a neurotransmitter that is closely linked to mood.

It helps with social behavior (basically, our ability to get along well with others). It also assists with appetite, sleep, digestion, and even your memory--many things that can assist in a happy, contented life. Interestingly, serotonin is one of the reasons that a little bit of junk food might seem like such a good idea when you are feeling sad--it boosts serotonin temporarily.