In Why we Love Helen Fisher looks at love from an evolutionary perspective. It's all about reproduction. Helen breaks this down further into three distinct types of love that are driven by different hormones.
Testosterone causes lust. We need to want to have sex in order to reproduce.
Romantic love is associated with dopamine and, interesting, low serotonin levels. Dopamine makes you happy. Whilst low serotonin can be associated with depression, it is also associated with obsessive compulsive disorder and this may explain why people in love focus on their loved one and nothing else. This feeling can last months or years.
Attachment on the other hand comes from the hormones vasopressin and oxytocin. These are responsible for the contentment we feel from long-term relationships and also the rush of love mothers feel for their newborn children.
These chemicals and feelings interact in interesting ways. Lust can lead to love and vice-versa.
Why we Love gives a biological basis for the complexities of love and love relationships. Fisher's model explains how someone could be in a committed relationship, whilst in love with someone else and at the same time have sexual fantasies about a third person.
Helen Fisher also gave a great talk on the topic at TED.
