14 “Show forth Your Mercy unto them that know You; Your Righteousness to them that are of a right heart”. As I have said, Those are of a right heart who follow in this life the Will of God. The will of God is sometimes that you should be whole, sometimes that you should be sick. If when you are whole God's Will be sweet, and when you are sick God's Will be bitter; you are not of a right heart. Wherefore? Because you will not make right your will according to God's Will, but wilt bend God's Will to yours. That is right, but you are crooked: your will must be made right to That, not That made crooked to you; and you will have a right heart. It is well with you in this world; be God blessed, who comforts you: it goes hardly with you in this world; be God blessed, because He chastens and proves you; and so will you be of a right heart, saying, “I will bless the Lord at all times: His Praise shall be ever in my mouth.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)