See, for me, the impermanence of relationships on Doctor Who is preventing me from fanning on it. I have the same problem with Joss Whedon shows, and with Lost -- I just need more stability than that. I can definitely enjoy a show without being fannish about it (and there are lots of examples) but in order to make the jump to fanning, I need those relationships to be at least somewhat stable -- they can change over time, but I just can't invest myself fannishly when they're always coming and going.