The second approach to avodah is [one of] humility and bitterness because of one’s distance [from G‑d]. A person following this approach is a1 “master of accounts,” [one who analyzes] every detail of his thought, speech and action, [evaluating them to discern those] which were not directed to G‑d not only things that are prohibited, but also those that are permitted but are not carried out for the sake of Heaven, for they are also evil.

Similarly, on the positive side, [such a person will also critically examine] whether his fulfillment of Torah and mitzvos is unsatisfactory. For each mitzvah has a specific purpose. For example, tefillin [is intended] to subjugate2 one’s mind and heart, [to direct] the mind and heart to G‑d alone. Similarly, the Four Species of the lulav3 should direct one towards the concept of unity, and the esrog [to the service of humility], its name (אתרוג) being an acronym for the verse,4 אל תבואני רגל גאוה “Let not the foot of pride come against me.” Moreover, there is a general intention present in all mitzvos to accept the Yoke of Heaven. (Note chapter 41 of Tanya which discusses both intentions.) If one does not fulfill mitzvos [with the proper intention] but instead performs them coldly, as a matter of habit, his Divine service is lacking. [When one realizes this, he will surely feel humble.]

This is particularly true when one meditates on the greatness and exaltedness of G‑d. In comparison with this, even genuine service has no value. Indeed, it is likened to sin. For example: serving a great king in a manner that does not befit his stature is considered demeaning, for each kind of service must be appropriate to [the station of] the one served; the service offered to a simple person cannot be compared with that given to a mighty king, and if offered to the king it would be considered sinful and rebellious, for it demeans his honor. The corollary of this analogy helps us understand [our relationship with] G‑d. In the face of his utterly unfathomable exaltedness, even the service of the loftiest tzaddikim bears no proportion whatever. Consequently, during the Ten Days of Penitence even such a completely righteous man as Moshe Rabbeinu has to recite the confessional prayers which declare, “We have transgressed…”; for considering G‑d’s true greatness and majesty, even true service is likened to sin. [This concept is alluded to in the comment of our Sages on the verse,5] “And he (Moshe) bowed to the ground [and prostrated himself].” They explain6 that he bowed down because “he saw G‑d’s attribute of truth” and who can stand before G‑d’s truth? (Note the maamar entitled Bayom HaShemini , 5651, [p. 75ff].)

[Nevertheless, G‑d does not demand such a standard of each individual.] There is a verse beginning,7 “When an ox or a sheep…,” [which prescribes what may be offered as a sacrifice. On this] our Sages comment:8 “When G‑d demands, He demands according to an individual’s personal abilities.” Since it is impossible for there to be an avodah commensurate with His true greatness, He asks only that each person’s avodah be true according to his own abilities. Nevertheless, periodically meditating upon this concept will bring about feelings of bitterness and surely so, when one contemplates the fact that one’s service is not true even in relation to one’s own abilities. This applies particularly if extraneous thoughts occur to a person during his service, or if he finds aspects of his speech or action that are not [in accordance with] G‑d’s [wishes]. For these distance him utterly from G‑d, and therefore cause him to cry out bitterly.

Summary:
The avodah of humility is evoked when one realizes that one is distant from G‑d and that one serves Him inadequately, especially in relation to His infinite greatness.