Apparently, 2 main genes are involved in the emotion we call "love" and pair-bonding (2 people or any other animal getting together and staying together, most likely raising a family, etc.): arginine viopressin (AVP) and AVPr1a. AVP is a message carrier and AVPr1a codes for a messenger protein called V1aR, which functions as a "catcher's mitt" for AVP.
Anyways, here's the interesting stuff! There are multiple variations for these genes and these actually do affect sexual and social fidelity! There are eleven different alleles for the two genes and these genes actually do regulate the strength of pair bonds. More AVP equates to stronger pair bonds and higher levels of fidelity.
But let's be careful here. Just because love is essentially regulated by two genes does not mean that that is all love is. If anything, I'd argue that it's even more of a miracle that such vast emotions and experiences can be caused by something so absolutely miniscule.
Here's the link to the article: