Why is it no matter how much love you give a man, they will still leave you?

(Generally speaking)
No matter how good you look, how much you take care of yourself, even be good in bed..
No matter how much you're independent woman and have a higher goals in life,
No matter how much you care for them, even cook for them, make them happy with little things
No matter how much intimacy, beautiful memories, care when they are vulnerable, affection and love...
They would still leave you, with not much explanation but simply you just lose your touch after time
"Not into you anymore"
They're just in love with you in the beginning and moments later they're bored.
Why?
Why is it no matter how much love you give a man, they will still leave you?
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