Fear does not seem like the most serious vice in the world. It never made the list of the Seven Deadly Sins. No one ever receives church discipline for being afraid. So why does God tell human beings to stop being afraid more often than he tells them anything else?
My hunch is that the reason God says "Fear not" so much is not that he wants us to be spared emotional discomfort. In fact, usually he says it to get people to do something that is going to lead them into greater fear anyway.
I think God says "fear not" so often because fear is the number one reason human beings are tempted to avoid doing what God asks them to do.
John Ortberg, If You Want to Walk On Water You've Got to Get Out of the Boat, Zondervan (2001), p. 118