Like sacramental actions, methods and gestures in prayer must also be meaningful, that is to say, the body must reproduce visibly what is taking place in the soul. As it is understood in the Bible, standing to pray is the bodily expression of the profound reverence of the creature before the exalted majesty of its Creator, in whose presence even the angels stand (Luke 1:19, "The angel answered, I am Gabriel. I stand in the presence of God, and I have been sent to speak to you.").
For the inferior stands up to greet the superior and remains standing as long as the latter is present. Thus, for example, Abraham stands before God when the latter speaks with him, knowing full well that he is only "dust and ashes". (Genesis 18:27)
The outward posture, however, does not only give bodily expression to the interior attitude, it also has an immediate effect upon this disposition. Without the effort of standing - and of the other prayer gestures...- our prayer will never attain the proper fervor, said Joseph Busnaya, but will remain "routine, cold and shallow".
~ Fr. Gabriel Bunge, Earthen vessels, pp. 145-146.
Thanks to Vicar Williams, The Orthodox Pathway, for this quote.