An interviewer asked Rabbi Yeshayahu Haber ztz"l [who founded an organization for people who need kidney donations]: "Why would somebody donate a kidney to a total stranger?"
R' Haber zt"l: "Because he is not a stranger, he is your brother".
He was מכוון to the Rambam ['מתנות עניים י' ב]:
לְעוֹלָם אֵין אָדָם מַעֲנִי מִן הַצְּדָקָה וְאֵין דָּבָר רַע וְלֹא הֶזֵּק נִגְלָל בִּשְׁבִיל הַצְּדָקָה שֶׁנֶּאֱמַר (ישעיה לב יז) "וְהָיָה מַעֲשֵׂה הַצְּדָקָה שָׁלוֹם." כָּל הַמְרַחֵם מְרַחֲמִין עָלָיו שֶׁנֶּאֱמַר (דברים יג יח) "וְנָתַן לְךָ רַחֲמִים וְרִחַמְךָ וְהִרְבֶּךָ". וְכָל מִי שֶׁהוּא אַכְזָרִי וְאֵינוֹ מְרַחֵם יֵשׁ לָחוּשׁ לְיַחֲסוֹ. שֶׁאֵין הָאַכְזָרִיּוּת מְצוּיָה אֶלָּא בְּעַכּוּ''ם שֶׁנֶּאֱמַר (ירמיה נ מב) "אַכְזָרִי הֵמָּה וְלֹא יְרַחֵמוּ". וְכָל יִשְׂרָאֵל וְהַנִּלְוֶה עֲלֵיהֶם כְּאַחִים הֵם שֶׁנֶּאֱמַר (דברים יד א) "בָּנִים אַתֶּם לַה' אֱלֹהֵיכֶם" וְאִם לֹא יְרַחֵם הָאָח עַל הָאָח מִי יְרַחֵם עָלָיו. וּלְמִי עֲנִיֵּי יִשְׂרָאֵל נוֹשְׂאִין עֵינֵיהֶן. הֲלְעַכּוּ''ם שֶׁשּׂוֹנְאִין אוֹתָן וְרוֹדְפִים אַחֲרֵיהֶן. הָא אֵין עֵינֵיהֶן תְּלוּיוֹת אֶלָּא לַאֲחֵיהֶן:
A person will never become impoverished from giving charity. No harm nor damage will ever be caused because of charity, as [ibid. 32:17] states: "And the deed of charity is peace." Everyone who is merciful evokes mercy from others, as [Deuteronomy 13:18] states: "And He shall grant you mercy and shower mercy upon you and multiply you." Whenever a person is cruel and does not show mercy, his lineage is suspect, for cruelty is found only among the idolators, as [Jeremiah] 3:42] states: "They are cruel and will not show mercy."
The entire Jewish people and all those who attach themselves to them are as brothers, as [Deuteronomy 14:1] states: "You are children unto God your Lord." And if a brother will not show mercy to a brother, who will show mercy to them? To whom do the poor of Israel lift up their eyes? To the idolators who hate them and pursue them? Behold their eyes are pointed to their brethren alone.
