"Truly God is good to Israel." Asaph knew that to be correct. But notice the qualifying statement in verse 1, "To those who are pure in heart."
As I read the Bible, many verses are foundational for me. One of those is Deuteronomy 29:4, "But to this day the LORD has not given you a heart to understand or eyes to see or ears to hear." Deuteronomy 29:2-3 tells us Moses was speaking to those who, with their own eyes, saw the LORD deliver the nation from Egypt by great trials, signs, and wonders. They still did not understand because God had not given them the necessary heart, eyes, and ears. Deuteronomy 30:6 adds to our insight of what God does for our heart, "And the LORD your God will circumcise your heart and the heart of your offspring, so that you will love the LORD your God with all your heart and your soul, that you may live." Another foundational verse to understand this heart God gives is Ezekiel 36:26-27, "And I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a new heart of flesh. And I will put my Spirit within you, and cause you to walk in my statutes and be careful to obey my rules."
We need one more passage to tie all this together. We find the reason we must have a God given heart in Jeremiah 17:9-10, "The heart is deceitful above all things, and desperately sick; who can understand it? I the LORD search the heart and test the mind, to give to man according to his ways, according to the fruit of his deeds." Instead of "sick", other translations say the heart is wicked, perverse, crooked, hopelessly dark and evil.
So we should not be surprised by Asaph's qualifying clause in Psalm 73:1. God is a good to Israel, uniquely to those with a pure heart. To those to whom he has given this gift of righteousness through the sacrifice of His Son. It is a gift of grace not a reward for anyone being worthy. Our responsibility is to protect and cherish what God has provided and to love the Giver with all our heart.
LORD God Almighty, help me to comprehend both the value and the cost of my new heart. May I never take either for granted.