Our limbic system is hard-wired for this: we have 2 basic desires -- high-fat food & sex/ procreation. These 2 'rewards' stimulate the dopamine response in a way that addictive drugs do as well.
Humans, being pair-bonding animals, not only find sexual relationships rewarding, we also find individual-pair-bonding rewarding. This is an evolutionary boon as it helps us stay together long enough to make a baby. It also helps us fall in love with the child, and want to stay with the child long enough to raise them to maturity. Indeed, having a healthy pair-bond is an almost universal sign of personal happiness.
Pair bonding animals have more D2 (dopamine) receptors, reward (I WANT THAT) sites, and non pair-bonders have more D1 receptors, satiety (I've had enough of that) sites. We all know how intense the feeling of love or infatuation can be, truly like a drug trip, and for some, very addictive.
As these neural pathways evolved at a time when we had so little in the way of stimulation, and now we have so much, its easy to see how easily we become over stimulated by things like video games, gambling, porn, internet, casual sex, drugs/ alcohol, etc. We're programmed to look for the next mood-enhancing moment, and we've become very crafty at finding it. We're hard-wired for lovin'.
Which leads back to men seeing, and wanting.
So we bond mostly because of the reward receptors we have, but also because we touch & orgasm (& breastfeed) which release oxytocin, another very sexy neurotransmitter. It not only makes us feel more trusting & bonded, but also helps reduce other cravings, like sweet cravings. Note tho, that without the "rewarding" dopamine, the oxytocin alone will not create the "in love" feeling. The two, like romantic partners!, must be balanced.
Women are not like men. Women can be mentally aroused and yet not get physically aroused. Their reward circuitry is based on their hormonal cycles, which are much longer than males'. Women tend to anticipate rewards more when their estrogen is at its peak -- pre-ovulation. This is easily attributable to evolution again: a woman should be more reward-driven (I WANT THAT) at the time when she is most likely to get pregnant.
Future posts -- what makes women attractive & the Coolidge Effect