The moralist and legalist pays his taxes and demands his rights from God. There are certain things God cannot ask of him. The gospel Christian has come to understand he deserves nothing good, and so relinquishes any concept of rights before God. In the gospel, there is nothing God cannot ask of us.
This is why the legalist cannot handle too much suffering. It is essentially a breach of what he thinks is the deal or contract between him and God.
Any insightfulness in these words should be attributed solely to Tim Keller.