Why MOST of men are...

I can't understand why most of men still have another one despite the fact they already have a faithful, loving ang caring girlfriend/wife. I observed this.
The big question is WHY
Is it because They are not satisfied?
is it because they are looking for fun?
is it because they are JUST MAN and its natural for them to do that.
Is it because they are not happy.
Is it because the are doing this because of lust.
Is it because they found themselves falling inlove to other
is it because of temptation.
And I can't understand also why most of girls too despite the fact they are cheating of there husband or boyfriend.they still want a relationship to that person
they always said and i always hear.
"I still love him"
"i can do everything to him"
"i can't live without him".
Is it called love and sacrifices??
I don't think so.
The big question is WHY

i said MOST not AT ALL WOMEN AND MEN ..

I WONDER WHY

as i always said and always believed its easy to be happy when you are contented to what you have because you are not asking or looking for more because you are CONTENTED right? anyway everyone of us has choices its up to all couple and person ..
Its just like this a person is blind because "today is a beautiful day but she/he cannot see... That person is blind, its a double meaning Did you get it??I Hope so.. I will further explain this . Dont get offend I just observed this though i don't have experience