Of Relationships and Cheating

Why do some persons who are already in a commitment cheats? even commitment as deep as sharing a marriage vow. I just can't get these questions off my mind as there are cases of infidelities I've come to know lately especially from people who are related to me,specifically related to me by blood. I was completely shocked,totally shocked when i  knew it. That person was the last person i expected to cheat, all along i thought that they have the best relationship, i felt really sad, i really didn't know about the real story but i just wish that both of them would patch things up and talk about it. I still totally wish that they'll save their marriage and work it out. What if i'll get married and discover that my husband is cheating on me,how would i react?..would i run?..would i end it right then and there?..would i save it?..would i be another Elin Nordegrens in the world?..am i ready for it?..as what my mother told me, i should be ready and strong enough when facing it,as there are really trials like that in marriage...pait..pait..pait kaayo...