Well, THAT'S a stupid question. A better one is, "Why wouldn't I be a fan?" Now THAT'S a hard question. Anyways...
I'm a fan because when I was, what was it, eight years old, I think, I heard "I Don't Know" for the first time. I got an iPod for my 14th birthday and I would borrow Ozzy CDs from the libraries, and put them onto my iPod. Ever since then, I loved his music. I like other bands, but Ozzy just tops them all.
PS If Ozzy is reading this himself, he should know that the song collecting has brought me to over 70 different songs on my iPod. Thank you for gracing this world with your awesomeness, Ozzy Osbourne.