What makes being faithful to your partner hard for most people?

Honestly, I don’t think it’s really that hard. I mean…. they chose to be with that person but, apparently, that person isn’t good enough. So, why waste time being in a relationship if you don’t plan to be faithful at all? If it’s hard being faithful, why not stay single and not hurt those who want something real?
What makes being faithful to your partner hard for most people?
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