People cheat, that's a fact. It's hurtful and sad, but it happens all the same. To figure out why humans risk love in such a way, Biological anthropologist Helen Fisher dug deep into the experience of love.
Fisher, in a TED Talk, had this to say about being in love -
"That person becomes the center of the world. You have an intense craving to be with that person, not just sexually, but emotionally. You can list the things you don't like about them, but all that gets pushed aside and you focus only on what you do like about them. It's an obsession."
In an experiment that she and a team of scientists conducted, they found that the brain reacts similarly to looking at a picture of someone they love as when it does when a person has an orgasm or takes cocaine. This means romantic love operates on motivation and reward.
However, there are actually 3 brain systems related to love. There's the above mentioned romantic love, which helps us focus our energy on one person. There's the sex drive, which make us search for a range of partners. And there's attachment, which is the feeling of security with a long term partner.
All of these systems are vying for first position in our brains, which is why we can feel attachment, romantic love and sexual attraction for several different people at the same time. Fisher said -
"It's as if there's a committee meeting going on inside your head as you try to decide what to do. I don't think honestly that we're an animal that was built to be happy - we're an animal that was built to reproduce. I think the happiness we find, we make."
Basically, we're capable of loving more than one person at a time, and that is why some people cheat.