It trips us up. It causes us to stumble. It can make us fall. And it's very frustrating. The result is that we spend all of our attention and energy on the rubbish and never accomplish the goal.
Frustration is a prime cause of discouragement.
Fear Another cause of discouragement is fear. The enemies of the Lord's work had struck fear in the hearts of God's people. Nehemiah himself saw their fear. They were afraid of the enemies and as a result felt like they could not succeed. They were ready to give up.