I have always known about Doctor Who because my dad grew up watching it. When I was sixteen I decided to give it a try, and I watched the first four episodes of the second series with David Tennant, starting with The Christmas Invasion. After watching those episodes, I didn't come back and watch more till I was seventeen. I have been a fan of the show ever since. I went into more detail about how I met Doctor Who here.
I'd say what made me a fan was the humor and the imagination and creativity that thrives in Doctor Who. It's one of the few science fiction shows that captures my interest and intrigues me. The well developed characters, inventive villains, brilliant plot lines, and unique concepts all meld together and make it a fascinating T.V. show. For me, however, the characters, deep emotions, and complex themes are the core of Doctor Who and run deep throughout it. It's also an important show because of how culturally significant it has become, and because it brings people together, not to mention it has been on-going since 1963. It's more than just a show.