Why do so many of us seem to want to change the person we fell in love with?

dude i have been with my bf for a yr and some months. we just graduated. i want to become a cop i used to smoke weed but now i stopped so i can prepare for the future. i then wanted my bf to change cause i did not want drugs in neither of our lives since i will be a cop i think it was for the better. he did chang to not for me but for him self. all i did was inspire him!!!!