Ecclesiastes 3:10 (ESV)

10 I have seen the business that God has given to the children of man to be busy with.

If you see that all your work does not really make a difference, it can torment you and almost depress you. Many things that are happening remain a mystery to you. You can't understand why they happen. Anyone who lives without Christ does not come to any better resolution than to characterize this inexplicable situation as meaningless. Outside of Christ you will lose all courage and hope. You see nothing but loose pieces of a puzzle. You don't see the purpose or the unity of life in what you experience and when you look around you.