It just sucks you in really. I've been aware of it for years but never really watched it growing up. I'd seen bits on tv here and there but after watching all of Doctor Who and then Sherlock I'd seen all the SuperWhoLock posts and though, "well, I'm 2/3rds of the way there, it'll keep me busy for a while, what the hell? I'll give it a go."
Instantly hooked.