Dvar Torah by Rabbi Zelig Pliskin
The Torah states, “An Ammonite or a Moabite shall not enter into the assembly of God, even the tenth generation shall none belonging to them enter into the assembly of G dforever. Because they did not meet you with bread and water in the way when you came forth out of Egypt” (Deuteronomy 23:4.5).
Rabbi Shimon said in the name of Rabbi Eliezer that from here we see the punishment of those who withhold kindness. During the forty years that the Israelites spent in the desert they had manna from heaven, quails and water from the well that went with them. In addition, protective clouds encircled them and journeyed before them to show them the way. In short, they lacked nothing. Nevertheless, courtesy requires that if people come from a journey, they should be welcomed with food and drink.
For failure to afford the Israelites this basic courtesy, the Ammonites and Moabites were banned from entering the assembly of the Almighty (they were not allowed to convert to become Jews). This involved the exclusion of the males of these two nations from marrying a Jewess even if they converted to Judaism (Yevomos 76b). From here we see the retribution of those who failed to show kindness to those who did not need it. How much greater will the punishment be for those who do not show kindness.