Well on the surface it does appear to be a shallow reason to split up. If you love someone then you should love them even if they have an extra bit of squish to them. However, I think that if you are watching someone you love really hurt themselves by their lifestyle choices and they will not listen to any help or encouragement to get better that you offer them then it is understandable to want to end the relationship.
I ended a friendship once because I was worried about a friend that was heavily into drugs and no matter what I did to help I couldn't get through to her... finally I had to say goodbye because I didn't want to watch her die.
If I had a problem, I would hope that my partner would be supportive and want to try help me out before up and splitting.