"You shall fear the L-rd, your G‑d"—Deuteronomy 6:13.
We are commanded to fear G‑d; never to be complacent, and to always bear in mind that transgressing His will leads to divine retribution.
This is also why we never say G‑d’s Name in vain, meaning, not in the context of a prayer or blessing – for this diminishes the fear and reverence we are commanded to harbor for Him.
