We have come short of living up to God's intentions for us. That is sin. When somebody else does wrong, we are usually pretty quick to criticize and to condemn. This shows that we know better than to do the same sort of thing. Wherein thou judgest another, thou condemnest thyself.
Sin does terrible things to people.
It makes innocent people suffer. It spoils the good things which God wants us to be able to enjoy. The only one who can save us from our sin is Jesus Christ. Neither is there salvation in any other. God sent His Son into the world to make it possible for sinners to get right again, and to stay right.