I think some people will feel this so called "fell out of love" because they want to replace and fill up their definition of love they wanted and used to feel, again. They maybe missed the happiness that their long term partner lacks of. If you're cheap, you'll find that happiness to someone else, but when you're real, you'll rest waiting for someone who sees your real worth, you value yourself more, but what's more better is that, you'll make time seeking reasons to come back with your long term partner, especially if it's really worth it. But when you're saying that you fell out of love and eventually you're with someone else after a short time of separation, that's cheating. You let yourself be happy and giving that someone else attention while in a relationship. Falling out of love is when a person has totally stopped loving their partner albeit how much they've tried to fix it, no third party involved. Those two are quite different.