Why do so many people engage in infidelity?

This reminded me of something my Mother said to me.
My father died when I was a teenager. He loved me and taught me as well, he was a great man and his walk matched his talk. My mother never did like me much but, I always knew it was her own failing, not anything I did. It didn't bother me.
One day after my father died, my Mom overheard me telling a new friend what a wonderful man my father was, how he worked so hard his whole life, how generous he was, etc. When I heard my mom, from another room say, vett , your Dad may not have been your Dad.
I just said back , well then if that was the case , I love him all the more , for loving me and taking such good care of me.

I knew I was a lucky child, and I knew he was my Dad. Thank You Dad, I love you.
My mom finally came around, even she couldn't resist this child, I love you Mom.