Why do some girls leave their ex for a new guy but then what their ex boyfriend back as soon as the new relationship fail?

Ex wants me back after leaving me for 6 months in pain for her friend. I begged and tried everything to win her back she cheated on the new guy with me then cut me off for 4 months and now wants me back. I don't know if the new guy knows she’s cheated or not a friend of mine did send him all of our text messages wit her admitting to some stuff but that was 2 months ago. I pretty much stop checking my Facebook account because I hate my ex for leaving me for antler guy and now wanting m back we dated for years
Why do some girls leave their ex for a new guy but then what their ex boyfriend back as soon as the new relationship fail?
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