We have a relationship with Christ in which our number one responsibility is to love him, but we also have a relationship with other believers, and our number one responsibility in that relationship is to love one another.
Joy is found in God Alone. God causes the Israelites to have joy and commit themselves to joyous praise. He teaches them where to locate their joy and how to have joy that isn’t based on feelings or circumstances but based on an everlasting relationship with Him.