Speaking to G-d will bring one to be embarrassed before Him with such holy connection that his face will shine

A person needs to accustom himself to speak between himself and his Maker, frankly with great and utter truth, until his heart is truly aroused, until he begins to speak with a passionate heart words of truth that are in his heart with great arousal, with repentance, until he sees his negligibility and the greatness of the Creator, until he is very embarrassed before Him blessed He. For until now he threw his sins behind his shoulders (-turned his back on them) and did not pay attention to them, and now that he acknowledges them, in this way he is inoculated with great embarrassment over the severity of his sins against a master and ruler, the essence and root of all the worlds, and this embarrassment is revealed on his face, and this shame is the aspect of the light of the tefillin, which are the sign of attachment (divaikus) Israel has with the Creator blessed He, and it is the aspect of (the) beam of light of the face, and through this shame all his sins are forgiven.

(Likutay Moharan, Torah 38; Likutay Aitzos, Embarrassment and Brazenness 2, Speech 7; Outpouring of the Soul 58).