John Piper on the Duty of Worship

"The real duty of worship is not the outward duty to say or do the liturgy.  It is the inward duty, the command: 'Delight yourself in the LORD!' (Psalm 37:4).  'Be glad in the LORD, and rejoice!" (Psalm 32:11).

"The reason this is the real duty of worship is that it honors God, while the empty performance of ritual does not.  If I take my wife out for the evening on our anniversary and she asks me, 'Why you do this?' the answer that honors her most is 'Because nothing makes me happier tonight than to be with you.'

"'It's my duty' is a dishonor to her.

"'It's my joy' is a honor.

"How shall we honor God in worship?  By saying, 'It's my duty'?  Or by saying, 'It's my joy'?

"Worship is a way of reflecting back to God the radiance of His worth.  Now we see that the mirror the catches the rays of His radiance and reflects them back in worship is the joyful heart.  Another way of saying this is to say, 'The chief end of man is to glorify God by enjoying Him forever'"  (John Piper, Desiring God).