“…for the striving of both makes one forget sin” implies that it is the struggle and the difficulty one is enduring that causes one to neglect sin. This is because man, who is composed of both a physical as well as a spiritual component, should be working to perfect himself in both areas. Torah is the perfection of man’s spiritual component. Derech eretz, which is man taking care of his body, his livelihood and his other material needs, is the perfection of his physical component. When man toils to perfect himself in these two areas, in his physical side through derech eretz, and in his spiritual side through Torah, then he will be free of sin. For he is toiling to achieve perfection, while sin would be an activity of destruction.
(This principle will be elaborated later for a clearer understanding.) Toiling in both areas means that man is striving for complete perfection. It is the struggle for complete perfection which leads him to neglect sin, for sin would impinge on the aspired perfection. But this toil does not cause him to neglect Mitzvoth (despite the effort he is expending) since the Mitzvoth themselves are for his perfection (which is what the struggle is all about).
But it would not be correct to understand this lesson according to the literal translation, that the (cumulative quantity of) effort expended when he is involved in two areas is sufficient to distract him from sin. For if this was the intent, then one could simply toil more in either derech eretz or in Torah in order to avoid sin. Furthermore (if it is simply the quantity of toil which is needed) it is impossible for a person to be toiling constantly, and there are always going to be times when he isn’t toiling in either of the two. If it is the constant toil which distracts him, then when he isn’t actually toiling he would be subject to sin!! But according to our explanation, the distance from sin is dependent on the person constantly striving to attain complete perfection. One who is toiling in both areas is involved (constantly) in the pursuit of complete perfection, and he is therefore distanced from sin (which would undermine his goal) even when he is (temporarily) resting (since that rest is not a deviation from his pursuit of perfection). (A deeper understanding of how striving for perfection insulates one from sin is coming in a few paragraphs.)
But if a person is toiling only in Torah, he is not involved in also striving to complete himself in relation to his physical needs. So he is vulnerable to sin, since he isn’t involved in pursuit of total perfection. As a human being made up of both a spiritual life force as well as a physical body, if he is only toiling in Torah, he is lacking what is needed to complete his physical side, and deficiency (sin) attaches itself to deficiency.
This is even more so when a person is not toiling in Torah but only in derech eretz, which leaves him an incomplete person (with the deficiency of sin even more drawn to the person who is lacking in his spiritual dimension).
Another reason why the distance from sin is made dependent on toil and striving is that sin is found in the place where a person is stationary (“yeshiva,” or “sitting”) rather than in a state of exertion and toil. This is alluded to by Chazal in Sanhedrin (106a). Rebbi Yochanan says: Wherever is says “vayeishev” (and he sat) it always means a language of distress. (Bamidbar 25:1) “And the Jewish people sat in Shittim…” followed by “and the nation began to stray (promiscuously)…” (Breishith 37:1) “And Yakov sat…” followed by the sale of Yosef. (Breishith 47:27) “And Israel (the nation) sat in Egypt…” followed by (v. 29) “and the days of Israel’s (Yakov’s) death approached.” (Melachim I 5:5) “And Israel and Yehuda sat in security” followed by (ibid. 11:14) “And G-d stirred up an adversary to Shlomo…” The Midrash (Shemoth Rabba 41:11) also teaches that whenever it writes “vayeishev” it implies a desecration, as it is written ((Shemoth 32:6) “And the nation sat to eat and drink, and they arose “l’tzachek” (lit. “to laugh” but which implies idol worship and promiscuous behavior).
This Midrash teaches the concept of “sitting” which causes man to stumble. The explanation is as follows. A person who is toiling and striving is demonstrating a lack of completeness (for this is what he is striving for). The person who lacks completeness is always in a state of potential completeness, moving towards the desired perfection. (He is said to “prepared for perfection.” It is his LACK of completeness that enables him to be capable of attaining more perfection.) Imperfection cannot be attached to one who is moving on the path to perfection. (It would contradict the essence of what the person is doing.) But a person who is static, behaving as if he has attained perfection, is precisely in a state to which imperfection is attracted. For in this (material) world, nothing can maintain a state of absolute perfection. So imperfection immediately attaches itself to anything which is perceived to be in such a state. This is in contrast to a person who lacks perfection but is struggling and moving towards that perfection. His movement towards perfection is exactly what insulates and protects him from further imperfections.
(To help understand this, imagine a person involved in constructing something. During the time that he is actually building the project, he cannot simultaneously act to destroy it. He may do things that are detrimental after he has completed it, or while he stops building it. But at the actual time that he is involved in building, he will not and cannot also be involved in demolishing it. It is the focused involvement in working to accomplish an unfulfilled goal that insulates a person from behavior which is destructive to that goal. While a person is involved in struggling for perfection he will not simultaneously undermine that pursuit of perfection through sin. This is the meaning of the Midrash and the Maharal’s explanation of it.)
This concept is also alluded to in Sanhedrin (91b; see also Breishith Rabba 34:12) where it is taught that the yetzer hara (force of evil) is given to a person from the time he leaves the mother’s womb, as it is written (Breishith 8:21) “For the inclination of a person is evil from his young days (“mi’neurav”). “Mi’neurav” is written with one letter “vav” missing, so the word can be interpreted as meaning from the time that he is “dislodged” from his mother’s womb, and it is then that he receives the yetzer hara. Why then? The yetzer hara is the force of desolation in the person, the lack that exists in the person which leads him to ultimate destruction and death. As long as the person has not entered the physical world, he is still in the process of “becoming.” During this process, lack and deficiency cannot attach themselves to him, since they are antithetical to the process which the individual is undergoing. Two mutually exclusive processes cannot go on at the same time in the same place. But as soon as the baby leaves the mother’s womb, the process of “becoming” has ceased, and the baby “is,” having come into existence. At that point the lack and deficiency which is a reality in every material creation attaches itself, and he is now on the road towards the ultimate deficiency. This is why the yetzer hara is also referred to as the “Malach Hamaveth,” the force of death.
(In a deep sense, the yetzer hara, which motivates a person to sin, is really the lack of security that exists in a person who is rooted in the physical dimension of his existence. Since the physical dimension is in fact a constantly deteriorating reality, from the moment of birth it motivates a person to behaviors that are directed towards covering up the deficiencies of the individual (egocentric and arrogant behaviors). This is in contrast to altruistic and humble behaviors, which are always directed towards attaining true and eternal perfection by using the material resources given by G-d to serve a purpose beyond ones own material existence.)