They are lonely? Can't make any friends ... got all bitter and twisted and bent out of shape. And they are not quite smart enough to realize why they have no friends. So they post here and reply to themselves because it makes them feel needed. Also, in that way they can hold a conversation on their own level. It's a brilliant strategy really, don't you think?
For example, the post with dead horse in the title, it has three replies but the original post and the three replies are from the same person. It seems that other people just don't want to communicate with this poster so they hold a conversation with themselves.