Why do guys always go back to their crazy ex?

I have a friend with this absolutely horrible girlfriend. His friends, coworkers, and family all don't like her because she's so awful. She's controlling and paranoid. At times he's even confessed to me that he can barely stand her. They break up every so often, but somehow end up back together even though his life is much better when she isn't in it. What keeps drawing him back?
Why do guys always go back to their crazy ex?
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