Rabbi Mayer Twersky
The Heresy of Hubris
So that his heart does not become haughty over his brethren and not turn from the commandment right or left so that he will prolong years over his kingdom, he and his sons amid Israel (Devarim 17:20)
The Torah alludes here to the prohibition of haughtiness because Scripture enjoins the king from haughtiness and conceit, and a fortiori others who have no case [for haughtiness] ... because haughtiness is repugnant and loathsome to G-d ... because greatness and exaltedness belong exclusively to Hashem (Ramban, ad loc.)
Rabbi Moshe of Covey authored the SeMaG, which enumerates and explains the six hundred and thirteen mitzvos. In a most stunning, remarkable passage he writes that upon completion of his oeuvre, he beheld a vision in a dream. He was told that "Behold, you have forgotten the foundational mitzvah not to forget Hashem and become haughty." Accordingly, he revised his work and featured the prohibition of haughtiness as mitzvas lo ta'aseh #64.
Rabbi Moshe Cordevero in his Tomar Devorah [chapter 2] characterizes humility, the obvious antithesis of haughtiness, as the key to all correct midos.
We certainly intuit that ga'avah is a deplorable mida. But a modicum of reflection and precise definition of terms are necessary to understand why it is so deplorable and why its antithesis, anavah, is so crucial.
A ba'al ga'avah is not necessarily one who exaggerates his abilities and accomplishments. Such a person is, in the words of my teacher Rav Gershon Zaks z"l, out of touch with reality! The ba'al ga'avah can be entirely accurate in his assessment of his abilities and accomplishments. He errs in feeling that "My strength and the might of my hand" (Devarim 8:17) are responsible for his abilities and accomplishments. The ba'al ga'avah feels independent and autonomous , and hence boastful and proud. An anav, by contrast, while appreciating the religious mandate for human effort and initiative, understands that Hashem has endowed him with whatever abilities he possesses, sustains him, provides him with opportunities and crowns his efforts with success. The anav can also be entirely accurate in his assessment of his abilities and accomplishments, but he recognizes his absolute dependence and all-encompassing indebtedness to Hashem. Accordingly, he is modest and unassuming.
With these definitions in mind we understand the emphasis upon anavah of the SeMaG and Rabbi Moshe Cordovero. The religious, theological need for humility and the heresy of hubris are codified in very stark terms by the Rambam (Hilchos De'os 2:3) "One who is haughty denies Hashem." Rambam establishes a rigorous equation between hubris and heresy. He is not merely guilty of something tantamount to heresy, but something which is heresy. Rambam quotes the same verse as the SeMaG. "And your heart will become haughty and you will forget Hashem, your God" (Devarim 8:14). Haughtiness, by definition, entails forgetting Hashem. One who is haughty feels independent and autonomous; this is a denial of Hashem, who is the source of our existence and sustains us.