This question of why we're equipped to love has already been answered via evolutionary theory: We love because we're meant to reproduce. Species continue through reproduction, and continuation of the species is paramount in evolution. Since mating is the ultimate goal, then feelings of romantic love are merely a vehicle toward this goal. Yet, the 2005 study found that the areas that cause sexual arousal in the brain aren't fully active as people fall in love. The two regions overlap, but the experiences aren't the same [source: APS].
This doesn't disprove the idea that love exists to foster reproduction, but it certainly raises new questions. Specifically, why do we continue to feel love even after we've reproduced? The current answer is also based in evolution: The combination of reward and attachment lead to a lasting addiction for a particular individual -- our partner.
Because of the association with reward motivation and its attendant releases of dopamine, that initial rush of romantic love resembles addiction rather than emotion. Over time, however, other neurotransmitters may play a larger role in forming long-term attachment that lasts beyond our reproductive years.
The chemicals vasopressin and oxytocin help humans and about 3 percent of other mammal species to experience lasting, monogamous love. These two chemicals are associated with our ability to form memories of others and help us recognize other people. They're also released, along with dopamine, during sex.
This combination of dopamine (which induces feelings of pleasure), oxytocin (which is associated with feelings of attachment) and vasopressin (which also promotes attachment and also allows social recognition) leads to a learned behavior where we actually become addicted to our mate [source: Economist]. Regardless of whether it's the sight of the person we're in love with or the injection of some drug, if both trigger similar releases, humans can experience both similarly and become addicted as well.
These same chemicals may also play a role in familial love, like that between a parent and child or among siblings. The chemical oxytocin, for example, plays a role in parental bonding. It's released in mothers during childbirth, and it plays a role in the production and release of breast milk [source: Johnson].
We experience love, then, to foster the relationships that may lead to reproduction and to maintain relationships with the offspring borne from those relationships.