Musar על ברכות 56:9

The primary aspect of fear of G-d is fear (awe) of His exaltedness. A person must think when he is engaged in prayer or performing a mitzva, that it is before the King of kings that he is praying or performing the mitzva. This is what the Tanna exhorts us saying: "when you pray, know before Whom you are praying" (Berachot 28b).