It's a pretty common myth, that people who have organ transplants can sometimes develop cravings or personality traits from the person who's organ they received. Like people who got heart transplants, falling in love with the wife of the man who's heart they received.

So I could imagine they become more confident and horny more often, and they think it's just the result of having a much bigger dick and balls, but slowly they become more of an asshole, and start seeing girls less like people we should respect, and more like worthless cum dumpsters to abuse and throw away.

Even when their new found confidence allows them to finally get with their crush, they find themselves acting like a prick to them. Then, when they start wanting to fuck their mom and sis, they try and get something done to get rid of this new personality, only for their crush to admit they would of never dated them before, and would dump them of they started acting all nice and polite.

Till eventually, they're corrupted so much, that all they can do is watch from the background, as their bully uses their body to fuck everyone that matters to them, then goes after all their friends girlfriends and sisters, becoming the most hated guy in town.