If It's just SEX, then why is he making it difficult to move on when, I simply want more, than just sex?

So this is a free question for anyone. But why would a guy find it difficult to move on from his friends with benefits who he has had for years. And this can be vice versa with the female to. But if it's just (SEX) I'm sure that he can find it elsewhere if he's not receiving it now lol. But if sex is just sex, and he's not really trying to commit himself to her. And if she decides that she wants to walk away, and stop the intimacy. Then why can't he just understand that and say okay and move on. Or better yet say if you have been at the FWBs with someone for a while. Why would it be hard to move on?
If It's just SEX, then why is he making it difficult to move on when, I simply want more, than just sex?
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