Luther spent much of  his time trying to find a gracious God.  Once God opened his heart to sinner being made righteous through Christ and His death, his heart was lightened and lifted.  He recalled the section of the Divine Liturgy when pastor says “lift up you hearts”, and we respond, “we lift them up to the Lord”.  Luther then said that we should always be lifting our hearts.

Therefore, singing, talking, and whistling without this lifting up of your heart to God are as much like prayer as scarecrows in the garden are like people. The name and appearance might be there, but the essence is missing.” ― Martin Luther, Faith Alone: A Daily Devotional