When a person has to force himself to sing to God and praise Him, no amount of effort will suffice.
But when the tongue speaks by itself, when the mouth and lips sing of their own volition, when he finds himself bowing spontaneously, then he is indeed able to acknowledge and praise God.
At that moment, a person and his prayer are not two entities, but one unity.
It is not that he recites prayer, for the prayer itself is praying, flowing from the song of his own nature.
The Talmud recounts that King David's harp would play by itself at midnight.
And the Jerusalem Talmud comments that in the verse, "When the musician would play music, the hand of God would be upon him," the word for "musician" should be read as "musical instrument" instead: the instrument would play by itself.
Why do our sages insist on conflating musician and musical instrument?
The answer is that when a person attains the perfect level of prayer, he himself is an instrument of song, of prophecy, of prayer.
The melody is his, and his entire being is none other than an instrument expressing that prayer.
Such self-expression – in which a person links himself not only to an inner state of holiness but also to the external structures of holiness – is prayer on the highest level.
Although such an achievement is exceedingly rare, we must make it our goal – we must know that such a level exists.
Even if such a state is not common, it can sometimes spark into flame from time to time in a psalm, a blessing, or even a word.
–Rabbi Adin Steinsaltz
From The Thirteen Petalled Rose, p. 137, by Rabbi Adin Steinsaltz
(with thanks to Rabbi Dov Bard)