Matthew, the author of this book wrote to a Jewish audience. Many of Jesus early followers were Jewish. As we look at the opening verses of chapter 6, Jesus speaks directly to three Jewish practices: giving, praying, and fasting. Jesus really challenges the hearts of those listening. Is what you do for righteousness or reputation? Is what you do a show to impress others or is it for you to draw near to God?
Jesus really goes after the heart. After motives. After intentions... It's a great reminder for us to consistently check why we do what we do.