We find many times in tanach and chazal (eg Sota 5a) where humbleness is praised and haughtiness is condemned. I would like to understand why that is. A person can conceivably fulfill all the commandments of the torah, and be a genuinely good person while still being conceited. What is it about excessive self-pride that is so incongruous with Judaism?
To every positive trait there is a flip side and vice versa. Character traits often have elements of good and bad to them. (Rambam:HaMada-Hilchos Deyos 1:3,4)
Giava AKA Haughtiness can be a path to piety as well as the undoing of it.
For example, if a person has so much pride as to say to themselves "I will not do this aveirah(transgression) because this is not who I am" then there is merit.
But, if someone acquires torah knowledge and wisdom and lords it over others as a status than that is an undoing of piety.
An example is when the Kohen Gadol (High Priest) Aaron would don the white robes to enter the holy of holies, he wore them as a symbol of humility, yet did not allow wearing them to give him a feeling of status as in "Look at me I get to wear the 'humble' clothes -which makes me special"
It's a slippery slope because Aaron knew who he was but he also did not allow it to manifest in his ego.
The ego is critical to creativity, self-expression and free will. Without those things we aren't complete people. We are supposed to want better things and be competitive but for the right reasons.
-But... let those things go awry and it can be almost idolatrous -(leading to having a quick temper which is compared to idolatry because a person forgets g-d orders things a certain way to every person) -and can lead to narcissism and hedonism.
I hope that helps
Arrogance is perhaps the worst character trait, because the greater one think one is, the less greatness one ascribes to G-d, and to others.
So the consequence of arrogance is thinking one's accomplishments are due to one's own efforts rather than to G-d, leading one to fail to praise and thank G-d. It also causes one to think little of other people, getting angry and impatient at them.
In addition, arrogance leads one to trust one's own ideas, rather than in Chazal and the great rabbis of this generation. This leads to heretical ideas and neglecting observance.
Finally, if one thinks highly of oneself, one is less likely to try to improve oneself through improving one's character traits, Torah learning, and observance.
I would say that the torah was given on har sinai the 'smallest' mountain to show that haughtiness and Torah dont go together. It is also no coincidence that moshe rabainu was the most humble of men and only through him could the Torah be given. The question was what is wrong with being haughty. So this answers it by saying he will not be able to learn Torah. Since torah and haughtiness dont go together.