Girls, how often has an ex, overreacted to someone you're "with"?

I was watching a show, and the guys ex, was "hanging out" with her cousin, since it really was her cousin, who had just moved, so she was showing him around. But her ex, got upset, and thought it was "another guy". It shouldn't have mattered to him, right? If they're ex's.. so, why did it bother him so much, even before the misunderstanding

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She wound up getting back with him, because she admitted she made a mistake
Girls, how often has an ex, overreacted to someone you're "with"?
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