Some people easily cheat their partners...

You should ask one of the cheaters. Someone who doesn't cheat can't answer this question.

May be they have some mental problem or they take it easy

The secret is not being able to truly love. People like this are users and they love only themselves and pleasure. If you have someone like this in your life you should get rid of them because they will never love you--or anyone else except themselves. And they will die lonely and alone, despised and hated by all the people that they hurt over their lifetime. Do not feel sorry for them, they deserve nothing from you.

Are you planning to do likewise? Sounds like you are looking for tips... People are insecure - in some couples, this keeps them together; in others, it drives the cheating you are asking about - sometimes by both partners, but usually one does the cheating, hurting the other partner as well as the person with whom they are carrying-on... But why, and how, I cannot answer, sorry.

A wise person look for a suitable partner than to cheat later in the name of their negatives.

Because they cheat themselves in the first place.