I have never cheated on anyone and don't imagine I ever will.
But I generally don't think that people cheat because they're unhappy with the person they're commited to - however, that may be part of it. I think often women use that a lot "I was unhappy with our relationship, I needed compassion and attention" etc, but I wouldn't say it's the only reason.
It's about chance, opportunity, lack of self-control, a "rush" and sometimes people are just selfish.

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To love is to suffer. To avoid suffering, one must not love; but then one suffers from not loving. Therefore, to love is to suffer, not to love is to suffer, to suffer is to suffer. To be happy is to love; to be happy then is to suffer, but suffering makes one unhappy; therefore to be unhappy one must love or love to suffer or suffer from too much happiness. I hope you're getting this down.