Expectations are the enemy

of serenity.  This past spring, I had a nice encounter with a man who had hiked the Appalachian trail.  I was helping at a continuing education event and we were discussing the expectations of the group that would be attending.  He shared the quote, "expectations are the enemy of serenity" with me.  He had met a man on the trail who had shared this quote with him.  Dreams, hopes and expectations. 
Is it bad to have expectations?  What is the difference between dreaming, hoping and expecting?