He is a jerk
I originally had this bullet titled with a much more adequate epithet for this type of man, but for the sake of civility, I changed it. Some men lose interest simply because they suck. Jerks will lose interest the moment a prettier girl walks by, if you gain two pounds, do not work out, say something silly, or even if you sneeze incorrectly.
These are the guys that make remarks in Cosmopolitan like, “Well, she could have been a 10 if she stopped eating so much.” Jerks will make you believe it was all you. In reality, it is all them, and they will never be happy.