I'm opinionated and generally forthright with people but I know I use self deprecation humour as an armoured shield from time to time.
Perhaps this gives the impression that others can say just what they like.
I'm not quite sure on the mechanics of such things
Recently I was reminded by someone, that I needed to loose weight.
I noted the concern but the statement was unleashed in front of others and it stung. It stung not because it was a correct statement........(I'm not an idiot) but because it was done publicly.
The problem with self deprecation is that it chaffs when someone else beats you to the punchline.