Humility due to one’s Mitzvos:[1]
The fact that specifically the physical Mitzvos have ability to quench one’s soul with the spiritual energy necessary to love G-d should bring a person to feelings of humility. It is specifically the lower and corporeal physical objects which bring man towards his ability in serving G-d, and hence one is to feel humbled, knowing that as great as the human kingdom seems to be, he is in need of the lowly physical objects in order to attach to G-d. This level of humility causes the individual to view himself as actual nothingness as is in truth the reality before G-d, as before G-d everything is nullified before Him. Accordingly, we can understand that which is written in books that humility is nullification and submission to G-d, as seemingly it is not understood why one needs to be humble to feel nullification of submission to G-d, as certainly one is a most low and dark creature in comparison to his creator. Rather, the intent of humility is that one be completely nullified before G-d to the point that he does not take up any space at all. This is the level of humility of Moshe which is higher than the humility of Avraham who stated that I am earth. It is this form of humility that leads to fear of sin, as will be explained next.
[1] Likkutei Torah p. 81b