People keep telling me I look really sad and I wish I knew why?

Without a photo we can only guess. Get a friend to go around with you and take pictures while you are doing stuff. Pictures taken when you are not aware of the camera will show you what you really look like.

I used to scare people and finally I realized it was my clip-on sunglasses. They had a gold colored frame that looked like devil horns between my eyes. I got rid of them. A long time later I bought a dark blue pin striped suit and that scared people. I don't know why, since I am a small guy. But I didn't argue, I got a light gray suit and then I didn't scare anybody.

In some cases, you may know why you are sad; in other cases, you may not. “You don’t have to be in a horrible situation to feel sad,” says 19-year-old Anna. “It can come on at any time, even if your life is trouble free. It’s weird, but it happens!”
Regardless of the cause—or even if there doesn’t seem to be one.
Write about it. When sadness clouds your outlook on life, you might want to try putting your thoughts on paper. In his inspired psalms, David sometimes expressed deep sadness. (Psalm 6:6) Writing about such feelings can help you to “safeguard practical wisdom and thinking ability.”—Proverbs 3:21.
Heather: “Writing helps me to organize the mental clutter that accumulates from sadness. When you can express your feelings and sort them out, the sadness is less overwhelming.
For more information on this subject and others, please go to jw.org. Also for free downloads, publications or read online. You can go to the search box for the book "Young People Ask answers that work" volume 1 and 2. Also the section on "Teenagers" on the website.