"Joy means sweet thoughts of Christ, melodious hymns and psalms, praises and thanksgivings, with which Christians instruct, inspire, and refresh themselves. God does not like doubt and dejection. He hates dreary doctrine, gloomy and melancholy thought. God likes cheerful hearts. He did not send His Son to fill us with sadness, but to gladden our hearts. For this reason the prophets, apostles, and Christ Himself urge, yes, command us to rejoice and be glad. 'Rejoice greatly, O daughter of Zion; shout O daughter of Jerusalem; behold, thy king cometh unto thee.' (Zech. 9:9) In the Psalms we are repeatedly told to be 'joyful in the Lord'. Paul says, 'Rejoice in the Lord always.' Christ says, 'Rejoice, for your names are written in heaven.'"
An extended quote from Dr. Luther
Commentary on the Epistle to the Galatians