People have always had shortcomings. We disappoint one another and oftentimes do not live up to our expectations as well as those of our mate. This by no means is not an excuse but rather a fact that no man or woman is perfect. Our goal however remains to strive for perfection if we are willing to commit to such a journey. With age comes wisdom and maturity. Much of what we do in our youth is done without giving any thought to consequences. Very often we hurt people because we are "self-centered."
If by "hurt" you mean (cheat or betray) those we love it is important to remember that cheaters don't want end relationships! The goal of any cheater is to (hold onto) all that is "good" in their primary relationship while addressing their other "needs" on the side. These needs vary from cheater to cheater (physical/sexual, emotional/romance, excitement of something new, feeling special or desired) are common reasons why people look outside of their relationships to "complete" their ideal life.
It takes courage to walk away from a "known" relationship and start off with nobody as one begins to pursue a new relationship. Some people treat relationships like jobs, they will not leave one until they have another one lined up.
The real interesting question to me is; Why do we ask why?
The truth is we really don't care about why! It's rhetorical or reflex type of question. The majority of us are not seeking any "real understanding" of a cheater's motives. In fact we've made up our minds there is no justification for hurting us! For many of us cheating is a "deal breaker" therefore it is nothing our mate can say that would cause us to say; "I get that and understand why you did what you did." Some people claim they're looking for "closure" but the reality is closure just means it's over and my self-esteem remains in tact. The reality is self-esteem is a gift you give yourself! If we need someone to rub our head or pat us on the back as we walk out the door it means we're still relying on (someone else) to make us feel good about ourselves! Everyone gets "their turn" for heartache and misery in one form or another. All we can do is keep living knowing there will be a tomorrow.