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Most people cheat due to their own personal insecurities. It's pretty often that they'll cheat because it felt good to be desirable in the eyes of someone else, so it makes them feel good about themselves, or because it was a nostalgic giddy feeling, having something new, and they didn't want it to stop, regardless of how hurt their partner would be. It's not always the fault of the significant other they're cheating on unless that person is making them feel extremely bad about themselves or being otherwise abusive. So it's on him and his issues, not you.