The primary chemical underlying romantic love is the neurotransmitter oxytocin. Oxytocin is released during orgasm for both sexes but also during childbirth or nipple stimulation for women. This helps along the biological functions of muscle contractions during birth or lactation for breastfeeding.
Experiments with prairie voles have shown that, when the gene for oxytocin is removed, this traditionally monogamous species loses its tendency for pair bonding completely. Scientists strongly speculate the same would happen for humans if oxytocin were blocked. Romantic love may be dependent on just a single brain chemical.
In males, vasopressin is also present, playing similar roles to oxytocin. It is thought that the period of oxytocin release is strongest in the first 18 months of romantic love, and trails off afterwards, though never vanishes completely. Sometimes oxytocin is whimsically called "the cuddle hormone".
Besides love, oxytocin has also been implicated in social bonds and trust in general. Synthetic oxytocin is available and some scientists have suggested it could be a sort of "social Viagra," and indeed experiments have shown that people are more trusting while under the influence of nasally administered oxytocin. Because it is implicated in both trust and love, some scientists have cautioned it could be used as a date rape drug. Like other neurotransmitters, oxytocin may be regarded as a "natural drug" — a substance, when released in the brain, which causes us to act differently, but is commonly accepted in human society because it has been around for millions of years.
Oxytocin also causes females to exhibit more mother-like behavior. This has mainly been observed in rats, but it is suspected the psychochemical response is the same in humans. The difference between someone who is considered your "type" for romantic love and someone who isn't is probably that the presence of one person causes the release of oxytocin and one person doesn't. This is the neurochemical basis underlying romantic love.
Dopamine, the neurotransmitter underlying pleasure in general, also plays a part in romantic love, and is released from environmental triggers from a good conversation to a kiss on the lips.