Reddit, why can't I seem to get along with people?

I'm a 17 year old senior and I barely have any friends. I don't go home and cry about it or anything like that, but I just want to know why. Whenever I try to talk to people, I smile and be as nice as I possibly can, but I can never build a lasting friendship. I can't make friends to go out and do things with so I usually just sit in the house and browse reddit. My cousins and I go to the same school and we'll meet someone new at the same time, but I'll be the only one in the group to get made fun of. I don't start ANYTHING with people, but I'm always a target for criticism. Even if I'm at an event with my cousins, a group of other people will always choose to mess with me out of the 4 of us. While my cousins (really my only friends) are making new friends, I'm continuously making enemies because I get tired of the way they treat me. I'm full of confidence.. or at least put a facade up while I'm in public. Why do I go through this. also, no comments like "man up and stop caring" because like I said before, I don't get all sad and depressed... I just want to know why.