In this article, Rabbi Michoel Gourarie answers [I believe based on the Lubavitcher Rebbe's explanation in Likkutei Sichos vol. 1 pg. 276]:
Humility is often erroneously associated with feelings of nothingness, self-negation, inferiority, unworthiness and being undeserving. However in truth, the consequence of these attitudes is not humility. On the contrary, feelings of nothingness lead to emptiness and insecurity. The person with these thoughts has no drive to achieve and feels unimportant, failing to recognize the significance in the good that he/she does.
In fact, false humility leads to arrogance. An insecure person is threatened by the views and perceptions of others, and needs to convince himself and others that he is worthy of approval. False humility brings egocentricity and self absorption.
Real humility can only be there together with a healthy sense of self. The humble person is fully aware of who he is. He acknowledges his talents, strengths and weaknesses. The humble individual feels safe with his identity and is confident and secure. On the one hand he recognizes that he has a part to play and that G-d acknowledges the importance of his good deeds. At the same time it is specifically this feeling of inner safety that allows him to: make space for others, understand that his strengths are G-d given and do not make him better than anyone else, feel secure enough with himself to understand another point of view with tolerance and respect, be discreet about his achievements without the need for great publicity..
That’s why G-d chose a mountain not a valley. G-d does not want you to be an empty valley with feelings of unworthiness and emptiness. He wants you to be a strong mountain – but a humble one.