If you haven't been to the doctor to rule out medical conditions, you could consider that. Iron deficiency could be an issue. I personally don't know much about your age group, my kids are under 4 yrs old. But my daughter was frail as the wind because she was Anemic. 2 weeks of an iron supplement cured it and she steadily gained weight ever since. Thyroid problems are also a large potential factor. You can ask for a blood test at your doctors office. Ask for your TSH and Iron levels. There might be other things your doctor could test for.
It may also just be your genetics. If your doctor rules out medical conditions and you are genetically thin, just try to find happiness with yourself. hardly any of us are truly happy with ourselves - there's always at least 1 thing we all wish we could change about ourselves. :)
Also, often time young people just don't eat well. Try to incorporate healthy "fat" foods (like avocadoes). Avoid the stuff that will wear down your body's natural ability to be as healthy as it can.
You are young, growing, and active so it seems like you are not gaining weight. It is ok you are not hungry 2-3 hours between meals. Are you a female or a male? If you are a male, you can continue to grow until 21-23 years old. If you are a female, you will start to slow down on your growing. Our family’s genetics do play an important part on our height and weight. The best you can do is make sure you eat healthy and are getting all the proper foods you need on a daily basis.