Channel Of Heavenly Abundance
בפסוק על אשר לא קידמו כו' בלחם ומים כו' בצאתכם ממצרים
On the pasuk concerning Ammon and Moav: "because they did not greet you with bread and water... when you came out of Mitzrayim" (Devarim 23:5).
The piece opens with the Torah's reason for excluding Ammon and Moav from the congregation of Hashem: they failed to greet Bnei Yisrael with bread and water as they left Mitzrayim.
פי' בס' קדושת לוי כי כשיצאו בנ"י ממצרים ונבחרו להיות סגולה מכל העמים ניתן להם דרך כל השפע הבאה משמים והם יכולים לקרב גם כל האומות
The Kedushas Levi explains that when Bnei Yisrael went out of Mitzrayim and were chosen to be a treasured nation above all the peoples, there was given to them the channel through which all the abundance that descends from Heaven flows, and they have the power to draw near even all the other nations as well.
Citing the Kedushas Levi, when Bnei Yisrael became Hashem's treasured nation at Yetzias Mitzrayim, they became the conduit for all heavenly abundance, with the power to bring even the other nations close to Hashem.
ועמון ומואב שלא נתנו זו הקדימה של לחם ומים לבנ"י לכן לא יבאו בקהל ה' ע"ש דפח"ח
And Ammon and Moav, who did not extend this gesture of greeting Bnei Yisrael with bread and water, therefore "shall not enter the congregation of Hashem" (Devarim 23:4); see there, for these are words that delight those who hear them.
Ammon and Moav refused to greet Bnei Yisrael with bread and water, and so they forfeited the chance to attach themselves and were barred from entering the congregation of Hashem.
כי לכן נק' גרים שע"י שמבטלין עצמם ונגררין אחר בנ"י יכולין ג"כ להתקרב
For this is why converts are called "geirim" (from the root meaning "drawn along"), because through nullifying themselves and being drawn along after Bnei Yisrael they too are able to come close.
A convert is called a "ger" because the word means to be drawn along; by nullifying himself and following Bnei Yisrael, he too is able to draw near to Hashem.
וכיון שרצו לקלל את בנ"י א"כ לא האמינו שבנ"י הם קיום כל האומות
And since Ammon and Moav wished to curse Bnei Yisrael, it follows that they did not believe that Bnei Yisrael are the very sustenance and existence of all the nations.
By hiring Bilam to curse Bnei Yisrael, Ammon and Moav revealed that they did not recognize Bnei Yisrael as the source through which all the nations are sustained.
ומיושב ב' הטעמים דלפי פשוטו קשה מאחר ששכרו לקלל אותנו מה נחשב שלא קידמו בלחם ומים ולפי הנ"ל הכל אחד:
And with this both reasons are reconciled: for according to the plain meaning it is difficult, since once they hired Bilam to curse us, of what account is it that they did not greet us with bread and water; but according to what was said above, the two are entirely one and the same.
This resolves a difficulty in the plain reading: if they already paid to have us cursed, why single out their failure to offer bread and water? The answer is that both stem from one root flaw, so the two reasons are really one.
Summary: The Sfas Emes, building on the Kedushas Levi, explains that when Bnei Yisrael left Mitzrayim and were chosen as Hashem's treasured nation, they became the channel through which all heavenly abundance flows into the world, and they possess the power to draw even the other nations close to Hashem. Ammon and Moav, by refusing to greet Bnei Yisrael with bread and water, declined this opportunity to attach themselves and were therefore barred from the congregation of Hashem, in contrast to the convert, the "ger," who is drawn along after Bnei Yisrael and so comes near. Their hiring of Bilam to curse Bnei Yisrael exposed the same flaw at its root: they failed to recognize that Bnei Yisrael are the very source of sustenance for all the nations. With this, both of the Torah's reasons for their exclusion are revealed to be one and the same.