It is because we are anonymous while on-line.

Just an aside.  It is also because others are anonymous to us on-line.  We seem to lack any perspective on them.  Its similar to the importance we give (at least emotionally) to downvotes compared to critical comments where we might even say "Oh yeah, I was wrong" (though not often, of course).  Anonymous downvotes and pseudononymous others are not the same thing, but it all seems related.

  p