כתב האריז׳ל (הובא במג׳׳א ס׳ מו)
"קודם כל תפילה יקבל עליו עול מצות עשה של ׳ואהבת לרעך כמוך׳. ויכוון לאהוב את כל אחד מישראל כנפשו ועל ידי זה כלולה תפילתו מכל ישראל לעלות ולעשות פועל."

רב שמשון פינקס: זה אינו בגדר עצה טובה בעלמא, אלא היא חלק מעצם סוגית התפילה. אדם שיש לו קפידא על איזה יהודי שבעולם זהו קפידה על הבן היחיד של השי"ת!
(ז) כַּמָּה מְעֻלָּה מַעֲלַת הַתְּשׁוּבָה. אֶמֶשׁ הָיָה זֶה מֻבְדָּל מֵיהוה אֱלֹהֵי יִשְׂרָאֵל... צוֹעֵק וְאֵינוֹ נַעֲנֶה...וְעוֹשֶׂה מִצְוֹת וְטוֹרְפִין אוֹתָן בְּפָנָיו... וְהַיּוֹם הוּא מֻדְבָּק בַּשְּׁכִינָה שֶׁנֶּאֱמַר (דברים ד ד) "וְאַתֶּם הַדְּבֵקִים בַּיהוה אֱלֹהֵיכֶם". צוֹעֵק וְנַעֲנֶה מִיָּד שֶׁנֶּאֱמַר (ישעיה סה כד) "וְהָיָה טֶרֶם יִקְרָאוּ וַאֲנִי אֶעֱנֶה".
(7) How exalted is the level of Teshuvah! Previously, the [transgressor] was separate from God, the Lord of Israel, as [Isaiah 59:2] states: "Your sins separate between you and your God." He would call out [to God] without being answered as [Isaiah 1:15] states: "Even if you pray many times, I will not hear." He would fulfill mitzvot, only to have them crushed before him as [Isaiah 1:12] states: "Who asked this from you, to trample in My courts," and [Malachi 1:10] states: "`O were there one among you who would shut the doors that you might not kindle fire on My altar for no reason! I have no pleasure in you,' says the God of Hosts, `nor will I accept an offering from your hand.'” Now, he is clinging to the Shechinah as [Deuteronomy 4:4] states: "And you who cling to God, your Lord." He calls out [to God] and is answered immediately as [Isaiah 65:24] states: "Before, you will call out, I will answer." He fulfills mitzvot and they are accepted with pleasure and joy as [Ecclesiastes 9:7] states, "God has already accepted your works," moreover, [G‑d] desires them, as [Malachi 3:4] states: "Then, shall the offering of Judah and Jerusalem be pleasing to God as in days of old and as in the former years."
יְקַבֵּל עָלָיו קֹדֶם כָּל תְּפִלָּה מִצְוַת וְאָהַבְתָּ לְרֵעֲךָ כָּמוֹךָ, וִיכַוֵּן לֶאֱהֹב אֶת כָּל אֶחָד מִיִשְֹרָאֵל כְּנַפְשׁוֹ, כִּי הִתְאַחֲדוּת בְּגוּפֵיהֶם שֶׁלְּמַטָּה גּוֹרֵם הִתְאַחֲדוּת וּדְבֵקוּת נַפְשׁוֹתֵיהֶם לְמַעְלָה, וְעַל יְדֵי זֶה גַּם תְּפִלּוֹתֵיהֶם מִתְאַחֲדוֹת. וְאָז בִּהְיוֹת תְּפִלּוֹתֵיהֶם כְּלוּלוֹת יַחַד, הִיא רְצוּיָה לְפָנָיו יִתְבָּרַךְ שְׁמוֹ (סִימָן צ"ב ובסידור).
(2) You should also resolve before each prayer to fulfill the mitzvah to "Love your neighbor as yourself." You must determine to love every Jew as yourself, because if there is, God forbid, dissension within Israel in this world, there is no harmony in the world above. However, when there is unity in this world it causes spiritual unity of our souls in the world above, and as a result, our prayers are unified; and when our prayers are unified, they are accepted favorably before Him, Blessed is His Name.
יערות דבש

כתב האריז׳׳ל שיקבל עליו מצוות ואהבת לרעך כמוך והיינו שימחול כל אדם וילך לפנים משורת הדין. כי הלא אצל הקב׳׳ה מידה כנגד מידה, ואם הוא מעביר מלבו כעס לרעהו ואינו רוצה לילך לפנים משורת הדין, איך ילך הקב׳׳ה לפנים משורת הדין? ולכן צריך ליתן בלבו מידת החסד והחמלה ואהבה ולאהוב שונאו ולדון כל אדם לכף זכות ואף יהוה יפן לקולו.
Tanya: Chapter 32
When you consider that all souls are matching and that we all have one Father.
That is why all Jews are called brothers—because they literally are brothers at the root of their souls in G‑d's oneness. It is only their body-selves that differentiate them.
It turns out that those who make their body-self their principal concern and provide their soul only a background role can never experience true love and brotherhood.
It's about Emunah. Hashemis endless kindness! And so it's simpler and clear that Hashem relates to every Jew like a father to his only son. If a person doesn't love his fellow Jew he also lacks Emunah inthe chessed of Hashem.
Rav Shlomo Carlebach:
How do you know you had a great tfillah? If after tfillah you want to hug every Jew in the world, then you had a good tfillah.
Rav Shlomo Carlebach
Who is the friend the Torah refers to when it requires you to love your fellow? Whoever you are standing next to right now!
(ג) מִצְוָה עַל כָּל אָדָם לֶאֱהֹב אֶת כָּל אֶחָד וְאֶחָד מִיִּשְׂרָאֵל כְּגוּפוֹ שֶׁנֶּאֱמַר (ויקרא יט יח) "וְאָהַבְתָּ לְרֵעֲךָ כָּמוֹךָ". לְפִיכָךְ צָרִיךְ לְסַפֵּר בְּשִׁבְחוֹ וְלָחוּס עַל מָמוֹנוֹ כַּאֲשֶׁר הוּא חָס עַל מָמוֹן עַצְמוֹ וְרוֹצֶה בִּכְבוֹד עַצְמוֹ. וְהַמִּתְכַּבֵּד בִּקְלוֹן חֲבֵרוֹ אֵין לוֹ חֵלֶק לָעוֹלָם הַבָּא:
(3) It is mandatory upon every man to love each and every one of Israel even as he loves his own self, for it is said: "But thou shalt love thy neighbor as thyself" (Lev. 19.19.). One is therefore, obliged to speak in praise of his neighbor, and to be considerate of his money, even as he is considerate of his own money, or desires to preserve his own honor. "But whosoever glorifies himself in disgracing his neighbor has no share in the world to come" (Yerushalmi, Hegigah. 2.1).
וְאָמַר רַבִּי יִצְחָק כׇּל הַנּוֹתֵן פְּרוּטָה לְעָנִי מִתְבָּרֵךְ בְּשֵׁשׁ בְּרָכוֹת וְהַמְפַיְּיסוֹ בִּדְבָרִים מִתְבָּרֵךְ בְּאַחַת עֶשְׂרֵה בְּרָכוֹת.
And Rabbi Yitzḥak says: Anyone who gives a peruta to a poor person receives six blessings, and whoever consoles him with words of comfort and encouragement receives eleven blessings. The Gemara explains: One who gives a peruta to a poor person receives six blessings, as it is written: “Is it not to share your bread with the hungry, and that you shall bring the poor that are cast out to your house? When you see the naked, that you cover him” (Isaiah 58:7). And the next verses list six blessings: “Then shall your light break forth like the dawn, and your health shall spring forth speedily, and your righteousness shall go before you, the glory of the Lord shall be your rearguard. Then, shall you call, and the Lord shall answer; you shall cry, and He shall say: Here I am” (Isaiah 58:8–9).
Rav Simcha Zissel of Kelm (Chochmah Umussar)
It is natural to envy the good fortune of others. The Torah requires you not to have negative feelings over the attainments of others, but to actually feel happy for their good fortune. It takes a conscious effort to work on feeling love for others. Whoever fails to work on this trait is likely to fall into the trap of feeling envious.
Likutey Maharan: 1:34
In every Jew there is something precious, a virtue not found in his friend...with this he can inspire his found to Avodas Hashem. His friend must acknowledge and accept this good point, as it is written of the angels: "They receive each from the other".
Hear your Father, your King saying:
Feel joy for the joy of my other children. Whenever you see or hear about someone else having some good fortune, feel joy.
Feeling joy for the joy of others is a potential source of joy that can be easily missed. When you feel joy for the joy of others, you will experience much joy in your own life.
When you realize that I am the one who has given that person the success or good fortune, it will make it easier for you to feel joy for the joy of My other children.
Feel joy when others are successful spiritually, financially or emotionally. Feel joy when others are successful in areas in which you wish to be successful, and also in areas that are not as important to you. Feel joy when others get engaged, married and have children. Feel joy when others get well after being ill. Feel joy when others get honoured. Feel joy whenever you see joy in another person's face. Think of the vast stores of joy that become available to you when you train yourself to have joy for the joy of others.
When you feel joy for My other children, I have joy in your joy.
