Both “joy” and “happy” have a fire in them. Happy is a blessedness in our head, meaning it is a subjective word. Happy has to do with intent and posture, and depends on external circumstances.
“Joy” also has a flame, but it’s a word of internal life and grace, divine breath, and light. It is an internal blessedness which rises from our insides. Nehemiah 8:10, “…joy of the Lord”.