I'm a fan (and im not going to say you should be too, as it was stated in the standard line when editing this post lol) because...

... it's very simple: I love the music As i got older, i never really stopped loving their music. They have the best voices, they're really talented singers as well as entertainers, and the songs are great. When i listen to a song, i can almost feel what they're singing, Its odd but true lol. Also i'm really into Close Harmony music, i love it. I can't explain how i feel when listening to Close Harmony, all those parts and voices make me happy in a way lol. And that's why I listen to the backstreetboys.