In my high school, college and then adult years, these comments were then followed up with other comments about my non-existent love life. When those negative kind of comments are then followed up questions about another sensitive subject, an already fragile ego is lead down the path of thinking that your weight is directly correlated to the lack of love life. I was led to believe that there was something wrong with me. That was my way of thinking my entire life until recently.
It was obvious from the passive aggressive comments, that this person was of the thinking that if I lost weight I would be happier and that I would find love. This was the type of person that should have been teaching me to love myself no matter what and to find happiness from within, but instead I was basically taught that my happiness lays in the hands of others and what they think. I know that these comments were made to me from their idea of love. They thought they were doing me a favor by making them (even though I asked, begged and pleaded with this person to stop making them). They thought that they could motivate me to lose the weight. Honestly, for a while I purposefully didn't lose any weight to spite this person. If they wanted me to lose weight so badly? Guess what... I gained weight just to piss them off. Stupid thinking, I know, but I was an unhappy and evidently spiteful individual who clearly wasn't thinking about my health (both physical and mental.)
My long and rambling story comes to this... I spent a lot of time worrying about what this certain person thought of me. I spent a lot of time unhappy because of it. It wasn't until I stopped caring what this person thought of me and my weight and started shedding the pounds for me and my health, that I started shedding my emotional baggage too. Now I'm the happiest I've been in a very long time.
I really believe that no one can make you do this but yourself. However, just a little bit of advice if you have a loved one whose weight you are concerned about. No matter how awkward, sit them down and have a frank conversation about it. Don't beat around any bushes. Tell them that you love them no matter what size they are, but that you are concerned for their health. Tell them you think they are beautiful inside and out, but you want them to stick around for a good long while and for that reason, and that reason alone, you wish they would do something about it.
Ask them to start living their life. In the words of a great blogger, ask them to Do Life.