Why do most people that cheat find it exciting and a rush?

The rush for is not getting caught & controlling everything & everyone around her, like you said it is a game to her

I had a friend like that too. It was all about the thrill of getting away with something.

Because these people are usually not happy at home. I am sure there is something that her partner is not providing. I am sure that there is something in her heart and life that she is missing. She may never find out due to the way she is going about things, but the rush that she feels in the moment makes up for all those lonely times she probably feels, for that emptiness that goes on inside her. It´s hard to understand unless you feel it. There are a lot of factors that contribute to this behavior.. I was her once, and I almost lost my family, and there is a man that is willing to loose his family and everything in his life for me, and I kind of wish I felt for him, but it was nothing more than a moment thing for me.. it is unfortunate, but unless she gets a wake up call this cycle may never stop, and a wake up call for her would have to be something very extreme.