לֹא־יָבֹא עַמּוֹנִי וּמוֹאָבִי בִקְּהַל ה...‘עַל־דְּבַר אֲשֶׁר לֹא־קִדְּמוּ אֶׁתְּכֶׁם בַלֶׁחֶׁם וּבַמַּיִם...
An Amonite or Moabite shall not enter Kehal Hashem ...due to the matter that they did not greet you with bread and water...(Devorim 23:4, 23:5)
The Ramban explains these verses in the following way: These two nations (Amon and Moav) owed appreciation to Am Yisroel because it was Avraham Avinu who saved Lot - the father of Amon and Moav. Hashem decreed that a person who comes from these two nations can never be allowed to a part of Kehal Hashem because they did not show the appreciation that they owed to Am Yisroel.
Look at how the Torah values the midda of hakaras hatov (appreciation)! People who don’t have that midda are not allowed to be a part of Kehal Hashem! Why? Because one of the central elements in the make up of a Jew is the midda of hakaras hatov.
