Anger is often the result of fear.

Fear is often the result of a threat.

Boss wrote something that you took personally, which you felt as a threat to your identity (e.g., you consider yourself a responsible person, but boss seemed to imply otherwise).

By not taking it personally because, if you realize, people do things because who they are, not who you are, that threat never materializes in your mind, you're less likely to become fearful and angry.

Also, you are less likely to be manipulated.

You should be proud of your self-control.