I asked him to quit smoking

I asked him to quit smoking many times since the last 4 years,he always listed and agreed to me and maybe quit for a few days but went back to smoking again...he has reduced the amount of smoking but didn't quit it totally in last 4 years totally... whenever he is in stress or something he starts again...he many a times hides this smoking habit from me and says he didn't smoked for months but in reality he keeps on smoking and keeps hiding it...i don't know how to make him quit or i don't know if he really loves me why is it so difficult for him to quit this for me??