You asked:
Why do we fall in love? — Anshul, Noida, India

Marshall Brain Answered:
Science is far, far away from completely understanding love. But there are a few things that we know for sure. And one of those things is that the human love bond – that feeling of falling in love and then sticking with a single partner – seems to be caused by a chemicals in the brain called vasopressin and oxytocin, along with the appropriate receptors for them. Very few mammals have the receptors, and those that do bond in long-term relationships. This article explains that the chemicals are released during mating, and if the proper receptors are present in the brain, a long term bond is formed. Prarie voles, which also have these receptors, mate for life.

what happens is that when the prairie vole mates, like humans, the hormones oxytocin and vasopressin are released. Because the prairie vole has the needed receptors in its brain for these hormones in the regions responsible for reward and reinforcement, it forms a bond with its mate.

When humans don’t have the receptors, they are unfaithful:

For more info see How love works.