Ultimately I would have thought it was a pretty good tool for weeding out potential 'friends' if they are going to make disparaging remarks about your weight.
The reality is that there is nothing wrong with being naturally thin. It is not unhealthy, no one (sane) thinks ill of you in any way, no one judges you or decides they'll avoid you because of your weight, no one makes assumptions that if there's a funny smell in a room full of people, it's the thin one, blah blah.
Maybe as teenagers you struggled with self-image because you had some unpleasant girls in your social circle, but as adults surely by now you can recognise that those people were just nasty, so their comments were meaningless, and not how society perceives you.
If you really want to change, it is really simple. You just have to eat more. And that can be one of life's greatest pleasures 
. A friend of mine who had always hated having no hips and boobs got some curves after having a baby, and eating more during and after, and for a bit she liked it, then she decided she felt 'fat', she was probably about a size 12, and barely trying, reverted back to her post-baby size, and has now accepted her size and appreciates she can eat what she likes. She hardly exercises.
I, on the other hand, generally eat less than her, go to a very vigorous Zumba workout 2, sometimes 3, times a week, have an active life, and still am a size 18. I just accept it. It's how it is. Now can everyone please stop worrying about their bloody weight and think about important stuff.