"I don't feel it" & "I don't feel it anymore." What does it truly mean?

Hi I got question to girls,

girls always saying "i didn't feel anything to him" or when they are breaking up "i don't feel it anymore" what this "feel" feeling really is?

it can't be love course love just don't disappear and if it is attraction then why it can be gone even the person often did not change (girls are breaking up for other reason) Can you girls help me up? I'm a guy and I try to understand this but I don't get this What this "Feel" thing really is.

Thank you