Every Morning before prayer, Chabad custom is to verbally accept upon oneself the Mitzvah of Loving your fellow Jew
הריני מקבל עלי מצוות עשה של ואהבת לרעך כמוך לרעך כמוך!
I hereby accept upon myself the positive commandment of 'To love my neighbor as myself!'
What is the connection between this specific mitzvah and prayer?
Before we answer that, lets explore a little bit more about this Mitzvah.
First, the Biblical source for Ahavas Yisroel.
(18) You shall not take vengeance or bear a grudge against your countrymen. Love your fellow as yourself: I am the L-RD.
The Sefer HaChinukh (“Book of Education”) is an anonymous work written in 13th-century Spain that clearly details the 613 commandments and explains the reasons behind them. Let's see how he explains this Mitzvah.
(א) מצות אהבת ישראל - לאהב כל אחד מישראל אהבת נפש, כלומר שנחמל על ישראל ועל ממונו כמו שאדם חומל על עצמו וממונו, שנאמר (ויקרא יט יח) ואהבת לרעך כמוך. ואמרו זכרונם לברכה (שבת לא א) דעלך סני לחברך לא תעביד. ואמרו בספרא (קדושים ד יב) אמר רבי עקיבא זה כלל גדול בתורה, כלומר, שהרבה מצות שבתורה תלויות בכך, שהאוהב חברו כנפשו לא יגנב ממונו ולא ינאף את אשתו, ולא יונהו בממון ולא בדברים, ולא יסיג גבולו, ולא יזיק לו בשום צד, וכן כמה מצות אחרות תלויות בזה. ידוע [גלוי] הדבר לכל בן דעת.
(ב) שרש המצוה ידוע, כי כמו שיעשה הוא בחברו, כן יעשה חברו בו, ובזה היה שלום בין הבריות.
(1) The commandment of love of Israel: To love [with] love of the soul each one of Israel - meaning to say that we have compassion for an Israelite and for his money, [just] like a person has compassion for himself and for his [own] money; as it stated (Leviticus 19:18), "you shall love your neighbor as yourself." And they, may their memory be blessed, said (Shabbat 31a), "What is hateful to you, do not do to your fellow." And they said in Sifra, Kedoshim, Chapter 4:12, "Rabbi Akiva said, 'This is a great principle in the Torah'" - meaning to say that many commandments are dependent upon it. As one that loves his fellow like himself will not steal his money, have adultery with his wife, cheat his money from him nor hurt him from any angle. And so [too,] are there several other commandments dependent on this - the thing is well-known [revealed] to all who have intellect.
(2) The root of the commandment is well-known - as in the way that he acts to his fellow, so will his fellow act to him. And there will be peace among the creatures with this.
היך עבידא הוה מקטע קופד ומחת סכינא לידוי תחזור ותמחי לידיה. (ויקרא יט) ואהבת לרעך כמוך. רבי עקיבה אומר זהו כלל גדול בתורה.
What is an example? Someone was cutting meat and laid the knife on his hand (and injured himself). Would he in turn cut the other hand? (Leviticus 19:18) "Thou shalt love thy neighbor as thyself." Rabbi Akiva says: This is the great principal of the Torah.
From the following passages in the Talmud we can see just how important the Mitzvah of Ahavas Yisroel really is and how severe are the consequences for the lack of it.
מקדש ראשון מפני מה חרב מפני ג' דברים שהיו בו ע"ז וגלוי עריות ושפיכות דמים
For what reason was the first Temple destroyed? Because of three sins which were committed there: idolatry, sexual immorality, and murder.
But the second Temple, where they were immersed in Torah, the commandments, and acts of kindness, why was it destroyed? Because there was baseless hatred there. This demonstrates that baseless hatred is equal to the three cardinal sins (idolatry, sexual immorality, and murder).
An enigmatic episode related in the Talmud regarding Ahavas Yisroel.
שוב מעשה בנכרי אחד שבא לפני שמאי א"ל גיירני ע"מ שתלמדני כל התורה כולה כשאני עומד על רגל אחת דחפו באמת הבנין שבידו בא לפני הלל גייריה אמר לו דעלך סני לחברך לא תעביד זו היא כל התורה כולה ואידך פירושה הוא זיל גמור.
There was another incident involving one gentile who came before Shammai and said to Shammai: Convert me on condition that you teach me the entire Torah while I am standing on one foot. Shammai pushed him away with the builder’s cubit in his hand. This was a common measuring stick and Shammai was a builder by trade. The same gentile came before Hillel. He converted him and said to him: That which is hateful to you do not do to another; that is the entire Torah, and the rest is its interpretation. Go study.
The question begs to be asked on Hillel's statement. Is it possible that all the other 612 mitzvot are only a commentary to this one Mitzvah? How is keeping Kosher only a commentary to Ahavas Yisroel? What about Shabbat and Tefillin and all the other Man to G-d based Mitzvot?
In the Tanya, the foundational work of Chabad Chassidus, the Alter Rebbe addresses this question.
(א) והנה, על ידי קיום הדברים הנזכרים לעיל, להיות גופו נבזה ונמאס בעיניו, רק שמחתו תהיה שמחת הנפש לבדה, הרי זו דרך ישרה וקלה לבא לידי קיום מצות "ואהבת לרעך כמוך" לכל נפש מישראל, למגדול ועד קטן. כי מאחר שגופו נמאס ומתעב אצלו, והנפש והרוח – מי יודע גדולתן ומעלתן בשורשן ומקורן באלקים חיים. בשגם שכולן מתאימות ואב אחד לכלנה, ולכן נקראו כל ישראל אחים ממש, מצד שורש נפשם ביי אחד, רק שהגופים מחולקים. ולכן העושים גופם עיקר ונפשם טפלה, אי אפשר להיות אהבה ואחוה אמיתית ביניהם, אלא התלויה בדבר לבדה.
וזהו שאמר הלל הזקן על קיום מצוה זו: זהו כל התורה כולה, ואידך פירושא הוא וכו׳
כי יסוד ושורש כל התורה הוא להגביה ולהעלות הנפש על הגוף מעלה מעלה עד עיקרא ושרשא דכל עלמין
Now when you will put into practice the above ideas, so that your body becomes "disgusting" in your eyes and "repulsive" and you find happiness into Joy Of Your Divine soul alone, this Frame of Mind will be an easily accessible, straightforward path to come to fulfill the Commandment, "love your fellow as yourself" extending love to every soul and Israel from great to small.
For when your body which separates you from others becomes "repulsive" and "disgraceful" to you, you appreciate more the value of the Divine Soul, which unites us all. And, as for the Soul and Spirit, who knows their greatness and worth, in the route and Source in the Living G-d? And ultimately at the level of essence, All Souls are equal being that all of them are alike and they all have one father and that's why all Israel are called Brothers literally due to their single source of their souls and the one G-d, and it's only their bodies that separate them. And therefore for those who make their body's primary and their soul secondary, it's impossible for them to have a genuine love and Brotherhood among them but only a "conditional love".
This explains Hillel the Elder’s statement concerning the fulfillment of this mitzvah: “This is the entire Torah, the rest is but commentary.”
For the basis and root purpose of the entire Torah is to elevate and exalt the soul high above the body, to [G‑d], the root and source of all worlds,
In summation: Through explaining Hillel's statement, the Alter Rebbe reveals to us a profound understanding of the Mitzvah of Ahavas Yisroel. When one is conscious of the collective soul and understands the integrated relationship of each soul, the separation of our physical bodies no longer causes indifference. When we connect with each other on a spiritual level we can love our fellow as we love ourselves, for in truth, we are all one soul.
The 'Tzemach Tzedek', the third Chabad Rebbe and grandson of the Alter Rebbe explains via a beautiful parable as to why the Chabad custom is every morning before prayer, to verbally accept upon oneself the Mitzvah of Loving your fellow Jew.
Tzemach Tzedek
"You shall be perfect with the L-rd your G-d." (Deut. 18:13)
Prayer is considered to be an offering before G-d - an offering of the soul. In the days of the Holy Temple, an animal offered for sacrifice had to be perfect and without defect; in the same way, when a Jew prays, he must also be whole of limb and without blemish. As all Jews are metaphorically part of the same body, if a person rejects his fellow Jew for whatever reason, it is his own self that becomes crippled. Therefore, it is customary to make the verbal declaration before praying: "I hereby accept upon myself the positive commandment of 'You shall love your neighbor as yourself."
Ahavas Yisroel, even to complete strangers hastens the coming of Moshiach!
The Rebbe
Brotherly love and harmony begin with the "great axiom of the Torah," namely, "Love your neighbor as yourself." As my father-in-law, the (previous) Rebbe would say: Love of a fellow Jew should even extend to a Jew on the other side of the world - even to someone you have never seen.
And when you have an opportunity to do a favor — materially or spiritually — for this Jew, you should do it freely and with a joyful heart.… When there is harmony amongst Jews (in addition to the harmony of a Jewish home — and preceding that, a Jew’s inward harmony — between his two inclinations), then there is peace and harmony in the entire world — especially, "peace in the land" — the Land of Israel.
We have seen that because of baseless hatred the second Temple was destroyed.
Let us see how through our baseless love the Temple will be rebuilt, speedily in our days!