A lot of people I know say they fear being alone, or being a virgin forever. For me, I've always feared dieing and never leaving my mark upon this world. I feared never being remembered. I feel like I'm destined to do something great and I would hate to not accomplish that. That might sound big headed or arrogant and I'm sorry for that, I'm finding it hard to word this so it doesn't sound like that.
EDIT: Oh and losing my best friend, I've only ever had one friend who actually understands me, he's like my twin brother, we both got into gaming when we were very young, both excelled at the same subjects, both of us took the same subjects for our higher education, both of us want to work in the gaming industry, both of us are internet nerds, we get each others humor, etc, you get the picture. Anyway, I'm a fantasy type guy, I like to think a lot, while my friend is a very down to earth kind of guy, he keeps me in check, if it wasn't for him I'd most likely be in an insane asylum, so without him my life would turn sour very fast. With all of that in mind, him moving house is killing me because I can't get in contact with him.