Rashi comments on the verse, “So that it will be good for you…”:
“If regarding a light mitzvah that involves no financial loss the Torah says, ‘So that it will be good for you and you will prolong your days,’ how much more so regarding the reward for more difficult mitzvos.”
It must be understood why Rashi changes the wording of the Sifrei. There it says that Shiluach HaKen is “a light mitzvah worth about an issar,” whereas Rashi says that “it involves no financial loss.” Seemingly, how can there be a disagreement about a matter of fact? We plainly see that the mother bird has some value. If so, in order to establish that all mitzvos have reward, one would seemingly have to learn from a mitzvah that involves absolutely no financial loss.
The explanation is as follows:
In the halachic approach, we learn from “You shall surely send” that one must send the mother bird away “even a hundred times.” If so, there is some financial loss involved, and therefore the Sifrei says that “it is worth about an issar.”
According to the plain meaning of Scripture, however, one may take the mother bird after sending her away once. Ordinarily, the mother bird returns to the nest even if the young are no longer there. Therefore, according to Rashi’s approach to peshuto shel mikra, there is no financial loss at all.
It follows that this mitzvah is the easiest mitzvah to fulfill. With other mitzvos, a person may sometimes be preoccupied with something else at that moment. Shiluach HaKen, by contrast, only becomes obligatory when he wishes to take the young, in addition to the fact that it involves no financial loss.
From the Wine of Torah:
Commands that are expressed in the infinitive form teach us that they must be fulfilled “even a hundred times.”
The explanation is that the act itself is limited, whereas the essence of the neshamah — the source — is unlimited. Therefore, its fulfillment is without limitation, “even a hundred times.”
At the beginning of one’s avodah, the arousal of the essence of the neshamah does not yet bring about a change in the revealed faculties. Therefore, one fulfills only a light mitzvah that involves no financial loss. And according to peshuto shel mikra, one need only send the mother bird away once, because the source has not yet brought about a change in the actual deed.
However, “one mitzvah leads to another mitzvah,” until the essence of the neshamah brings about a change in the revealed faculties — “a hundred times.”
Thus, from a light mitzvah one proceeds to the other mitzvos mentioned there: a new house, a vineyard, and a garment — representing the distant surrounding level, sustenance, and the close surrounding level.