When you are psychologically and emotionally healthy, you don’t think about yourself. However, when an emotional problem vexes you, then your self-consciousness may push everything else out of your mind, and you may be totally focused on yourself.
If you are in severe physical pain, chances are that this occupies your attention so completely that you cannot focus competently on other matters, whether with regard to work or relating to other people.
You may ask for time off from work or apologize to people because you cannot relate optimally to them. Much the same may happen when it is you yourself who is uncomfortable. To the degree that you focus on yourself, to that degree you may be distracted from work or other people. However, in contrast to physical pain, you may not even notice that you are being drawn to focus on yourself. Consequently, you may not even be aware that you are not functioning up to par.
Take the time to read the whole thing, you won’t regret it – it’s very short. I think I’ll buy the book.