What are the reasons for you to cheat to your partner?

I actually cheated on a partner once and I'm with the guy now that I cheated on him with. For me, he hadn't necessarily cheated but he goes on sites and talks to girls and cybers with him so therefore it felt like he had cheated. We tried to mend things but they never really got mended before he decided without telling me, that he was going off to the Marines. We argued the entire time up to a day before he left so while he was away it didn't feel like we were together. There were a ton of things not sorted out, a ton of questions unanswered, and I didn't know if he wanted to be with me because he was away and wanted someone to write to him or because he actually loved me. While he was away I met someone that showed he cared and was everything that I was hoping the guy would be so I needed up seeing him and we slept together a few times but he knew I was with someone and that I wasn't going to tell the guy until he came home. In my mind, the ex was already getting medically discharged and depressed and I knew he didn't need to hear the news. Once he got home I know I made the right choice due to the fact that he didn't want to go back to school and was going to be living in a place and wanted to stay there the rest of his life, that I refused to live in. He's more of a country person whereas I've lived near cities my entire life and want to work in one like DC so it wouldn't work. Our futures were different.