The Chinuch explains the root of the mitzvah of shikchah: it trains us to have a generous eye and a giving heart, and not to feel that every last thing belongs to us. The practice of agricultural release shapes the character of an entire people.
מִשָּׁרְשֵׁי הַמִּצְוָה. כִּי הַשֵּׁם בָּרוּךְ הוּא רָצָה לִהְיוֹת עַמּוֹ אֲשֶׁר בָּחַר, מְעֻטָּרִים בְּכָל מִדָּה טוֹבָה וִיקָרָה וְשֶׁיִּהְיֶה לָהֶם נֶפֶשׁ בְּר[וּ]כָה וְרוּחַ נְדִיבָה. וּכְבָר כָּתַבְתִּי (מצוה טז) כִּי מִתּוֹךְ הַפְּעֻלּוֹת, תִּתְפַּעֵל הַנֶּפֶשׁ וְתִהְיֶה טוֹבָה וְתָחוּל בִּרְכַּת הַשֵּׁם בָּהּ. וְאֵין סָפֵק כִּי בְּהוֹתִיר הָאָדָם חֵלֶק אֶחָד מִפֵּרוֹתָיו בְּשָׂדֵהוּ וְיַפְקִירֵם שֶׁיֵּהָנוּ בּוֹ הַצְּרִיכִים, תֵּרָאֶה בְּנַפְשׁוֹ שׂבַע רָצוֹן וְרוּחַ נָכוֹן וּמְבֹרָךְ, וְכִי הַשֵּׁם יִתְבָּרַךְ הִשְׂבִּיעוֹ בְּטוּבוֹ וְגַם נַפְשׁוֹ בְּטוֹב תָּלִין, וְהַמְּאַסֵּף הַכֹּל אֶל הַבַּיִת וְלֹא יַשְׁאִיר אַחֲרָיו בְּרָכָה שֶׁיֵּהָנוּ בָּם הָאֶבְיוֹנִים אֲשֶׁר רָאוּ הַשָּׂדֶה בְּקָמוֹתֶיהָ וַיִּתְאַוּוּ תַּאֲוָה אֵלֶיהָ לְמַלֵּא נַפְשָׁם בָּהּ כִּי רָעֵבוּ, יוֹרֶה בְּנַפְשׁוֹ בְּלִי סָפֵק רֹעַ לֵב וְנֶפֶשׁ רָעָה וְגַם רָעָה תְּבוֹאֵהוּ, וּכְמוֹ שֶׁאָמְרוּ זִכְרוֹנָם לִבְרָכָה (סוטה ח, ב) בְּמִדָּה שֶׁאָדָם מוֹדֵד, בָּהּ מוֹדְדִין לוֹ. וְזֶה הָעִנְיָן יַסְפִּיק לָנוּ עַל צַד הַפְּשָׁט גַּם בְּלֶקֶט וְשִׁכְחָה וּפֶרֶט הַכֶּרֶם וְעוֹלֵלוֹת.
It is from the roots of the commandment that God, blessed be He, wanted that His people that He chose be crowned with every good and dear trait and that they would have a blessed soul and a generous spirit. And I have already written (Sefer HaChinukh 16) that through the actions is the soul acted upon and it becomes good and the blessing of God descends upon it. And there is no doubt that when a man leaves a portion of his fruits in his field and he abandons them so that those in need should benefit from it, he shows about himself a satisfied will and a correct and blessed spirit, and that God, may He be blessed, has satiated him with His good, “and his soul will also recline in the good.” But the one who gathers it all into his home and does not leave over blessing that the destitute — who saw the field in its fullness and had a desire to fill themselves with it, because they were hungry — not benefit from it, shows, without a doubt, about himself an evil heart and a bad spirit and evil will come to him. And it is like they, may their memory be blessed, said (Sotah 8b), “In the measure that a person measures, so will he be measured.” And this matter will suffice for us on the level of its simple understanding also for gleanings, forgotten sheaves, fallen grapes of the vineyard and bunchless grapes.