What sucks you into Fandom?

I've been poorly today and overwhelmed by my to do list, but I've been pondering something that I'm not sure there is a definitive answer to, so I'm just going to explain and ask.

I'm falling head over heels for two new (for me) programs and one I used to watch but has sucked me in. I'm talking about GCB, Once Upon a Time and Army Wives.

Now here's the part I don't get. I have no, absolutely none at all, desire to even look for fanfic or delve into their fandoms. Now I had fanfic plotted in my head and was searching for LJ Comms before I had even finished watching the H50 pilot. I'm equally committed to watching NCIS:LA and short of a wee foray into their fic which I didn't like I'm not at all interested in their fandom. Likewise for Bones, Castle and anything else I watch casually.

My question? What is it about some programs that suck us in and make us want to practically absorb fandom?