Well I think most people are lucky to be skinny. It's not that those people didn't have to work for it. It's just that most people have bad genetics or are just in bad situations. I know I have a slightly slow metabolism. And most young people don't have to work for it. Sure, they exercise, but even without the exercise they'd still be skinny.
Some people are jealous, and like to assume you just got lucky. It's an excuse for some people to think "Oh I am stuck at this weight. I'm not lucky, like that person." I think it would help if you explained how hard you work to stay thin, or say "It's not luck."
You are lucky because others can't no matter how hard the work at it. Some people don't have the determination and the perseverance to loose weight. You are lucky. Take the compliment with humility. Others will NEVER have your luck in this topic.
A lot of the time, genetics (and even age) play a huge roll in how 'skinny' a person can get. There are people with larger frames and physique who can put in as much effort, but will NEVER attain the same results as someone who is smaller and naturally thin.
So .. in a sense .. you are very fortunate to be able to attain what you have accomplished.