There's an intoxicating effect of just contributing to a conversation, saying something interesting to other people.

But there's also shyness and paranoia: what if whatever I say makes me look bad? What if it comes back to screw me over later?

The Internet dilutes the latter a bit because you're anonymous. But you can still get caught e.g. if someone goes through your post history and recognizes you.

Some people are very paranoid and don't like to share anything because they're afraid somehow it will affect them later. I guess on the other hand, there are people willing to share confidential information for attention without protecting themselves adequately (or at all!).