Why would you cheat on your partner/spouse

I was wondering... why is it a usual thing for people to want to look at the grass on the other side as greener? Its kind of a shock to me to find people who have been dating to for donkey year, all of a sudden to find that their partner is actually cheating on them or has not been faithful to them... Or a married couple and the husband just realizing that he loves someone else or the other way round...

I pondered for a long time, and i found that the only things standing between long term happiness and relationship break-down is .... are you ready for this?... OPENESS & TRANSPARENCY. To sum it into 1 word TRUTH. A lot of relationship lack truth and trust but i guess the root is truth...

Thinking of it i really wonder why it is so hard to tell things the way they are... In the first place if you had told the truth about how you feel about her/him you would have save your self the stress or heart ache of breaking up. Well this is just my thought WHATS YOURS? Thanx for reading.