once you pop (the fic) you just can't stop

Why is it that there are some fic where I know they are OOC, and about as interestingly plotted as a dictionary, and with as much realism in people's actions as a chicken has teeth, but I feel compelled to keep reading anyway?

Why?

(This question brought to you by the fact I read a couple of hundred thousand words of a particular series at the weekend, and I have absolutely no idea what the reason was.)