In this reading I think that CS Lewis emphasized that we have a right to pursue happiness as far as it follows Natural (moral) Law, and legal law. I agree with Lewis but I think that he could have made his argument stronger if he directly linked God into this passage. I think rather than Natural Law we should think about God’s Law. We can pursue happiness as long as it does not contradict God’s Law.
Our purpose on this earth is to praise the Lord in all that we do. If a family member dies we should praise Him because He has taken another saint under His wing and brought them up to their true home in heaven. We are called to be happy in our praise to him. I think that this is the only way we can have happiness, because God. is the sole source of happiness. If a child of God has a loving relationship with his wife that makes him happy, it is only because God gave him that wife. God should be praised for such a wonderful gift.
Even when you are in a horrible situation you are called to be happy, to be otherwise is a sin. As you may recall Paul and Silas spent time in prison together and happily praised God all the while. We cannot show discontentment with what God has given us, this includes the trying times we all must face in life. God will never place anything in our lives that we cannot overcome.
The question is whether we have a right to happiness. I do not think that we do. We are all sinful human beings, and the only thing any of us deserve is hell, let alone happiness. However, because Christ died on the cross and took all of our sins away we have been given a right to everything, from happiness to eternal life.