What does it serve in an evolutionary sense? Why would your mind cling to one who left you? It makes sense that your mind would get rid of them fast to go find another mate. Women do move on faster to be in the safe protection of a new male.
However both men and women are prone to getting stuck and obsessed with people that dump them.

1) What does this serve biologically?

2) As far as psychology goes, is this mostly due to ego? If you don't feel your self value threatened, would you still obsessed over someone dumping you just because you still desire them?

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Thanks.