Through the Torah and Mitzvot [commandments] that a person does, they bring upon him thoughts and desires to want to purify himself. This is called “He who comes to purify.” This is the time when he exerts in order to purify himself from self-love.
At that time, he comes to a state of sorrow and pain at not being able achieve purity, but to the contrary. This is so because each time, he sees more of the truth: He is immersed in self-love and does not see any possibility in the horizon that can bring him out of this.
Then when he regrets this, he acquires a need for the Creator’s help. At that time, the help from above comes, as our sages said, “He who comes to purify is aided” (Shabbat 104a). This is regarded as “all the light.”
At that time, we say, “According to the sorrow is the reward,” meaning that which he regrets not having, he can obtain this for his sorrow. It is like a person who regrets not having respect. Therefore, his sorrow makes him do things that will earn him respect.
Therefore, when one regrets not being able to purify himself from vessels of reception, what will be the reward, so he will have the strength to purify himself from vessels of reception? It follows that “According to the sorrow is the reward,” meaning on whatever substance he regrets, on that substance he receives the filling.
Thus, the question was, How is it permitted to receive reward? The answer is that the whole prohibition on receiving reward is that he will be a receiver. Yet, here it is to the contrary; he wants the reward of being able to bestow, and this is certainly permitted. It follows that there is no light without a Kli [vessel], so it is impossible to draw mercy if there is no sorrow.
“According to the sorrow is the reward” means that he is rewarded for that which he regrets. He is not regretful over money and is given respect. The reward of being able to bestow is permitted because this is the purpose of the work.