We want people to focus on what we do right. Our problem is that we don't always do right. We reject God's way. What James is not saying. If you've broken one, you might as well break the rest.
All sin is the same.
Where sins are the same - they each separate us from God. Though to differing degrees. It's possible to be almost totally good. James is setting up a hypothetical situation. Radio commercial - girl calls her father.