Why do some women only what their ex back after their new relationship has failed?

Anonymous
I have notice some girls will leave their ex partner for someone else once things get hard then only want that person back after all there new relationship fail. Why is that? I have one friend that cheated on her boyfriend for a guy she thought was perfect a year later he become toxic and she's been begging for her ex back every single time a new relationship of her fails. She says she loves him regrets leaving him during his depression but he wants nothing to do with her besides a friendship.
Why do some women only what their ex back after their new relationship has failed?
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