I believe it may be a little bit bragging, but is mostly validation-seeking. Many people just seem to be wired in a way that craves a social response to their behaviour. Call it neediness, insecurity, whatever...I think the kindest thing is to simply identify it as a personality trait.
Back in the slide show days I remember being impressed by the stance my parents took which was to sit back and enjoy the holiday snaps as much as they could, because the person showing them was getting something out of that.
Today, I will act interested in a dream a co-worker wants to tell me, not because I am particularly gripped by how they "were flying, but also not, and everyone's face was Graham from accounts", but because it seems to make them happy to have someone listen. If simply clicking a "like" button or posting a thoughtful comment can give someone warm fuzzies (and we know it does) then I'll do it.