Why do infidelity exist in so many marriages?

Awaiting the release of my first novel" Trained To Cheat" release date to be announced. I decided to post my first blog, so bare with me.
Men and women have to deal with the subject of Infidelity on a daily basis. Why do we as a people feel a need to have more then one person in our lives to fulfil our needs. Cheating goes deeper then just a mere desire for the hunt, or to know if I still have it going on. It is deeper than just getting revenge for all the men that have mistreated you in the past, or all the women that did not realize that you was one of the good guys. Cheating could be a form of behavior that is passed down through generations or maybe it is a form of learned behavior. Whatever the reason,
Why do infidelity exist in so many marriages?
Share your thoughts.

Craig C

Husbands, Love your wives, just as Christ Loved the Church, and gave himself up for her, Eph 5:25

Make your wives the first priority in your life. Love, loyalty, trust and respect are necessary for emotional well-being. Wives who are treated as number one priority usually make their Husbands feel the same way.

I Am Working Toward This!!!!