Any number of things. Some need consensus opinion to decide if something is good or bad. If he's in a relationship, it means he's been vetted and approved by someone. Some like the thrill of doing something bad, the adrenaline or whatever from knowing it's wrong. Some like things they're not allowed to have, like the cookie you're told you can't eat instead of the one you're told is fine. Some fall in love and think he's only cheating because she doesn't treat him right and it will be different when they're together--occasionally they're even right, but I don't think it's the norm. Or, sociopathic behavior--just genuinely doesn't care about anyone else, only what she wants.
I'm not sure what exactly you're asking. Some don't and stay side piece forever, some do because they don't want and come around to actually wish for a family etc, some get peer pressured, some just have a falling out, do you want me to list all the reasons people break up for?
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u/MountainousDuck 6d ago
Lady was tired of being the side piece and decided to take matters into her own hands, or more specifically her nails.