Serotonin is a somewhat counterintuitive hormone to make the list, since it actually promotes feelings of calm and contentedness. But it's possible to chart the lifespan of a romantic relationship by tracing the roller coaster ride of serotonin in the brain. During the early attachment phase of love, serotonin takes a back seat, residing at low levels, while other reward-regulating chemicals take over. As a result of that serotonin dampening, people become borderline obsessed with their beloved, unable to focus or eat whenever apart [source: Lite]. Eventually, once a relationship solidifies, the raphe nucleus in the brain stem begins to cook up more serotonin, eliciting those warm and fuzzy feelings of togetherness that typify longer-term attachment. The only downside of that serotonin upshot is the loss of excitement, also colloquially known as the end of the "honeymoon phase."