First of all, there are enough episodes to not get bored. Usually, after I haven't watched 'hated' episodes in a time, I even really like those again and could watch them a couple of times.
Then there's the different layers. Right now, for example, I'm watching out for the foreshadowing.
You can watch with different people, and especially if they are new to the Buffyverse or a specific episode that can provide fun. For me there's also the understanding more and more. Like, I've started this Buffy reference thread some time ago, where I posed some questions about references I didn't get. Stuff like that (always discovering something new) can add enormously to the hookedness.
Then there's fanvideos. Maybe this sounds dumb, but some are so frickin awesome that I finish watching and just think 'man, I love this show...lets watch some Buffy'.
Last but not least, the whole show is just awesome.
EDIT: And how could I forget...what adds to the awesomeness is that, although this show deals with monsters and whatnot, I always find something to relate to. And that's just really cool.