As an old livejournal hand, I too had a really hard time getting into tumblr, but once I had a dashboard with enough content, hitting the refresh button became scarily addicting. I'm still a little frightened sometimes of just how much time I spend there when the content is 90% crap or repeated gifs or what have you. I guess I just keep hitting the refresh for those occasional gems. I think there are studies of rats doing something similar with food pellets.