I've done it for years.
What makes it worse is I have a compulsion to answer/help people, so i've done it a LOT!
I've always spend 3 hours typing a helpful, accurate, detailed answer, then looked at it and deleted. (not only once either)

Perfectionism is one part. (OCD)
Fear of getting wrong and causing someone harm if they carry it out is another (for me)
Another is afraid come off stupid/don't know what talking about.

I just signed in to forum to ask a question about my compulsion, and I answered this first, lol. (gotta laugh)