When there’s no physical or sexual contact, your brain fantasizes more about the person in a sexual manner. If you feel emotionally close, you’ll eventually crave physical intimacy and fantasize about it. Also, when you don’t interact with someone like your partner, but find them emotionally compatible within the little time you spent… you just feel happy around them. You never got a chance to entirely know them, and romanticize them from afar. You find them more intriguing than your partner and your heart refuses to limit itself in your current relationship… which defines cheating.