Self doubt of what?
Hummm...
Doubting yourself.
Perhaps anything about yourself.
...can I rise to the occation
...did I make the right choice
...am I doing the right thing
For some people who suffer severely the feelings can invade even the most basic parts of daily activity.
Really there is not much need to inquire why someone is feeling self doubt most of us have felt it at some level at some point in our live and will again.
Haven't you, Gladfly, ever wondered with a little concern about something you've done or want to do?
That is a degree of self doubt. When those thoughts and concerns happen until a degree of anxiety surrounds our feelings about ourself then the kind of self doubt worth hating has set in. As has been mentioned empathy would be the ideal course of action when some one expresses this has happened.