Humans are not a monogamous species. It's hard wired in our brains to go out and mate with as many people as possible. It is theorized that 7 years is about all we can take before the hard wiring instinct kicks in and we go in search of greener pastures. In other words, it's our id that cant handle the monogamy and our superego that keeps us from straying (providing one has a well developed superego). Always a battle going on inside the brain.