Well, I think that when you are in a good relationship, there just isn't a whole lot to talk about. I could start posts on how much I love my dh and and how fantastic he is, but the only thing anyone could is say is "aw, that's sweet" and move on to the next topic. Some people may not want others to think they're bragging too.
While there are a lot of negative threads, I don't think their popularity is all due to people liking drama. I do think that there are a lot of people who genuinely want to help.