Guys , what is the most things make an ex girlfriend unforgettable?

she is stuck in your mind , and you will never ever forget her , what're the reasons?
is it her beauty?
her tenderness? honesty?
being all ( mom, friend , sis , mistress , etc ) in one?
or any other reason
what do you think?
Guys , what is the most things make an ex girlfriend unforgettable?
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