This isn't so much a philosophical as neurobiological topic, but there is plenty of philosophical impact.
Romantic love: a mammalian brain system for mate choice
These data contribute to our view that romantic love is one of the three primary brain systems that evolved in avian and mammalian species to direct reproduction. The sex drive evolved to motivate individuals to seek a range of mating partners; attraction evolved to motivate individuals to prefer and pursue speciﬁc partners; and attachment evolved to motivate individuals to remain together long enough to complete species-speciﬁc parenting duties. These three behavioural repertoires appear to be based on brain systems that are largely distinct yet interrelated, and they interact in speciﬁc ways to orchestrate reproduction, using both hormones and monoamines. Romantic attraction in humans and its antecedent in other mammalian species play a primary role: this neural mechanism motivates individuals to focus their courtship energy on speciﬁc others, thereby conserving valuable time and metabolic energy, and facilitating mate choice.
And notice, as an attempt at a humorous aside, that love isn't only for the birds.
So, we have:
Sex drive (testosterone) -> Attraction (dopamine) -> Attachment (oxytocin)