The downward escalator of life
The two sons of Adam, Kayin and Hevel, took two different paths in life. Kayin became a farmer, and Hevel became a shepherd. At some later point, Kayin decided to bring an offering to Hashem. He brought from the worst of the fruit of his land. Hevel was also inspired to bring an offering. Unlike his brother, he brought the best of his cattle. Hashem accepted the offering of Hevel, but ignored the offering of Kayin. This caused Kayin to become depressed. Hashem, as a form of encouragement, taught Kayin a lesson. He told him that if he improves, then he will cheer up. However, if he doesn’t improve, then sin crouches at the door. What does this lesson mean?
A verse in Proverbs may give us a clue. It says: the way of life is for the intelligent to go upward; this is to avoid the grave below. What this means, is that people are forever mobile. Unlike Angels, who are static, a person is always on the move. They can’t stay in one spiritual level. They are either on the rise, or they will fall. The intelligent, those that care about their spiritual future, will work hard to constantly improve themselves. Otherwise, they may end up below, in the spiritual grave.
This is exactly the lesson that Hashem was teaching Kayin. If you improve, as in if you achieve perfection with good deeds, then שאת. While this could be translated as “cheering up”, it literally means “to rise”. Hashem was telling Kayin that he would consequently rise in his spiritual level. However, Hashem told Kayin that if he doesn’t improve, then he will surely fall in sin. Even if this descent isn’t palpable, as a Human it is impossible to remain static. There are only two options: upward movement, or the opposite. It is similar to standing on a descending escalator. If you do not make the effort, you will move downward.
 Based on Be’er Yosef to Genesis 4:7
 Ibn Ezra ad. loc.
 Genesis loc. cit.
 Ibid 4:2. Rashi ad. loc. explains Hevel knew that the Earth had become cursed due to the sin of Man, so he turned to a more lucrative profession
 Ibid verse 3
 Rashi ad. loc. This is inferred from the lack of description of what he brought, unlike the glowing description given to Hevel’s offering (Mizrachi ad. loc.). Also, why else would Hashem not accept Kayin’s offering? (ibid)
 Seemingly his inspiration was from his brother Kayin. This should have offered Kayin some merit, similar to Yehudah who inspired Reuven to repent (see Rashi to Deuteronomy 33:7). I’m curious that if there was some merit, how it manifested. Cf. Radak ad. loc. who entertains the possibility Hevel brought his offering first, and Pirkei D’Rabbi Eliezer Chapter 21 which says Adam instructed his children to bring offerings
 Genesis 4:4
 Ibid verses 4,5
 Ibid verse 7
 Proverbs 15:24
 Vilna Gaon ad. loc.
 הולך. See Vilna Gaon’s Imrei Noam to Berachos 64a where he says that most people are mobile only in this world, but in the next world, since they can no longer fulfill mitzvos, they become static. This isn’t true for special righteous individuals, who can ascend even in the next world. This is what the gemarra means when it says תלמידי חכמים אין להם מנוחה לא בעולם הזה ולא בעולם הבא. Rav Dessler also says this in Michtav MeEliyahu I pg. 286. The source for this is Zechariah 3:7
 עומדים. See Isaiah 6:2, Zechariah loc. cit., Nefesh HaChaim 1:10, and Ruach Chaim 6:1
 Gehenom can be referred to as שאול (Eruvin 19a)
 This could be the meaning of the Zohar I pg. 54b