“Be mindful with a minor Mitzva [commandment] as with a major one, for you do not know the reward given for them.”
During the descent, when the Torah and Mitzvot [commandments] are minor in his eyes and he does not feel any importance or preciousness about them, he thinks he has no reason to engage in Torah and Mitzvot because his work is in vain and that his work is worthless, since he does not find any rhyme or reason in it. Therefore, what point is there in exerting uselessly?
Our sages said about this that one should be mindful with observing them the same as with a major one, as though the Torah and Mitzvot are a grave matter to him now, meaning in their full importance, for then he could make efforts because he knew how to appreciate them.
They said about this, “You do not know the reward given for the Mitzvot.” That is, a person does not know the reward for Mitzvot, if he engages in Torah and Mitzvot that he regards as important, or even if he does not regard as important, and yet he overcomes and does them.
Perhaps there is more benefit in this overcoming that he does during the descent than during the ascent, since one cannot know which state brings more contentment above.
This is why they said, “Be mindful with a minor Mitzva, from the words [in Hebrew], “trifling and contemptible.” A “major” one means that he feels the importance of Torah and Mitzvot, for then is the time when he can engage above reason, for the reason of the descent always mandates the opposite of the truth. In other words, it should be said that he can gain more from the lowly state if at that time he overcomes.