What happens when a perceived Alpha gets into a relationship and then starts to display some Beta behavior?

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I see this quite a lot. The 'Alpha' gets the girl and starts a relationship, but then Beta behavior starts to creep in; they get a little needy, a little too emotional etc. You see that low level of disgust from the woman every now and again (that hypergamy starts to kick in).

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What I never really understood is why women continue to stay with that guy? Is it simply that this guy is more Alpha than his current competition? Is she just too emotionally invested to break up? Or is it that he's not showing enough Beta behavior for it to be real enough issue for her?