It is biological, and exists in non-humans. The first famous (and famously cruel) demonstration of this was in experiments by Harry Harlow involving infant attachment to mothers in monkeys:

http://en.wikipedia.org/wiki/Har...

The experiments were crude, but showed that the baby monkeys preferred a soft comforting surrogate mother dummy that provided no milk, to a cold, metal one that did.

Since then, the ideas have gotten a lot more refined, and a lot more is known. For example, many species (like ducks) "imprint" on the first caregiver they encounter as "Mommy."

Love in the sense of romantic love, as a precursor to sexual relationships is also biological. It does not exist in species that do not form extended pair-bonds, but does in species that do. It's biological correlate appears to be characteristic neurchemistry during the attachment-formation period. Chemicals such as oxytocin seem to be involved.

For humans, this is a good starting point:

http://discovermagazine.com/2003...

Despite the inherent difficulties, a number of recent studies have shed

light on the brain chemistry of human love. Like those of the

monogamous prairie vole, human oxytocin receptors are located in several

dopamine-rich regions of the brain, suggesting that oxytocin is

embedded in our reward circuitry. One study compared the brain activity

of people looking at pictures of loved ones or at pictures of

non-romantic friends. The pattern of activity in the cortex was markedly

different depending on which type of face the subject was exposed to.

FMRI scans of brains processing a romantic gaze bear a striking

resemblance to the brain activity of new mothers listening to infants'

cries. They also resemble brain images of people under the influence of

cocaine.

The face-recognition studies are of particular

interest because a number of animal studies have convincingly linked

oxytocin to the formation of social memory. One hypothesis is that

oxytocin release during key pair-bonding events like sexual climax or

childbirth helps cement the image of a partner or a newborn in the

mind's eye. Mothers who breast-feed their children often describe

powerful memories of infants gazing up at them during nursing. The

vividness of those memories, and their association with warm feelings,

may well be the imprint of oxytocin.

Beyond human, try Matt Ridley's "The Red Queen" to get oriented.