(1st of all i wanna say i'm spanish so maybe someone don't understand me, i can only say i'm sorry about that, if that happens)
I'm a fan (and you should be too) just because of their amazing lyrics and their really great songs!!! their songs always make me feel happy, calm, relaxed, excited or just making me wanting to meet them and tell them all... what i think about you!!! (depending on the song of course!)
Also... you just have to see how long they've been together and the time they'll be!!!!
Well... the truth is that i don't really know how to express why i'm a BSB fan in words, i think the best way to express it it's giving them all my support, love and passion in their concerts (as i did in the Madrid's concert on 23rd April 2008), and of course if i'd meet them it'd be so much better!!!!!! 
So i'm their fan just because how they are, how they sing (perfectly), how they dance...
and i'll be their fan 4EVER as well as they'll always be UNBREAKABLE!!!!! and LARGER THAT LIFE!!!!!!!
BackStreet's Back! Alright!
Elena Blasco, Madrid, Spain