Chazal tell us (Rashi to Bereishis 10, 9) that Nimrod "knew his Master and intentionally rebelled against Him." How could it be that Nimrod acknowledged Hashem's omnipotence and, nevertheless, rebelled against Him?
Rav Wolbe explains that the concept of "knowing one's Master and rebelling against Him" was more prevalent in the earlier generations because they had a clear perception of Hashem from their earliest years. It was precisely because of his closeness to Hashem that Nimrod had a problem. It was difficult for him to maintain such a lofty madreiga for an extended period of time. Had someone asked Nimrod if he believed in Hashem, he would have answered, "Absolutely!" and, nevertheless, his impulses refused to act in keeping with this knowledge.
Likewise, when a person ascends a rung on the spiritual ladder, there is an automatic feeling of resistance. Rav Dessler compares this concept to a spring. The harder one presses down on the spring, the harder it will bounce back toward him. The bigger the resolution one accepts upon himself, the more resistance he will feel and the harder it will be for him to maintain this new madreiga.
When one learns mussar and engages in serious self introspection,he will be surprised to find that he, too, possesses (to a certain extent) these feelings of resistance toward ruchniyos. One such example is our behavior after Yom Kippur, when many people sing and dance. Is this happiness borne out of genuine feelings of gratitude to Hashem for pardoning our sins? One who learns mussar will discover that it is more likely that this singing and dancing is an expression of joy at being relieved of saying so many tefillios andconfessions. It is simply too difficult for us to maintain the lofty level of Yom Kippur. This is a subconscious act of rebellion against Hashem!
The Alter of Kelm writes that Chazal's statements can be compared to stars and the way to properly "see" them is with a "telescope" i.e. the study of mussar. Though the above statement of Chazal might seem "distant" and difficult to understand, the study of mussar acts as a "telescope" for it allows us to discover these very traits inside ourselves and thereby understand the profundity of Chazal's words.
Thus, it is imperative to progress spiritually with small steps since there is less resistance. Someone who decides to completely refrain from speaking by accepting a ta'anis dibbur for a day, and thereby prevent himself from speaking lashon hara and other prohibited speech, may find that the resistance is too great. The next day he might end up speaking twice as much as he had until now! Additionally, one must make an effort to internalize his awareness of Hashem, for then it becomes not mere "knowledge", but part and parcel of his very being. The result will be that he will act in consonance with his knowledge instead of rebelling against Hashem.