When objectively looking at monogamy from an evolutionary stand point, it doesn't seem to make much sense. If the idea is to have as many offspring as possible and carry out your lineage, why would humans only mate with one person?
In 2013 two researchers were unknowingly working to solve this mystery at the same time and surprisingly published 2 very different studies the same week. Here TIME has compiled this information and offers a brief synopsis of each theory. The final verdict is still out on why we consider it the norm to stick with one mate, but one thing is for sure: it had nothing to do with love.