I wonder what it says about the human psychology that people feel that they "have to" digg, submit, friend, comment, etc. You can use Digg just like you use HN

I wonder what it says about the human psychology that people feel that they "have to" digg, submit, friend, comment, etc. You can use Digg just like you use HN - just browse for articles, read them, perhaps comment or not. I always found this interesting when points or achievements were in a game - there is a subset of people who feel that they "have to" do those things, even if they are not fun and don't really want to. They get mad at the designers for making things less fun when the entire thing is optional even to the general gameplay of the core game. Actually the more I think of it, the friend system is the only thing that HN doesn't have that Digg has. Or maybe I just haven't looked that closely when I visit Digg lately.