Descendants of Amon and Moav are disqualified from marrying into the Jewish people. Two reasons are given by the Torah for this prohibition: they didn't bring bread and water to the Jewish people as they traveled near their land, and in addition, they hired Bilam to curse them. We can understand why the attempted curse of Bilam was a reason to distance ourselves permanently from those nations responsible for hiring him. If Bilam had succeeded, it would have meant the annihilation of Bnai Yisroel. It is difficult to comprehend, however, why the lack of hospitality shown by Amon and Moav should be sufficient grounds to prevent us from ever marrying them.
The significance of Amon and Moav's inaction can be understood on two levels. When Eliezer was searching for a wife for Yitzchok, he was looking only for one character trait: kindness. It was the kindness that Rivkah exhibited by giving water to a traveler that convinced Eliezer that Rivkah was destined to marry Yitzchok. Chessed is so essential to the success of a marriage that the absence of it would disqualify a potential wife for Yitzchok, regardless of any other virtue she may possess.
It was a total lack of consideration for others that eventually led to the exclusion of the descendants of the Givonim who joined Bnai Yisroel in the days of Yehoshua. Chazal (Yevamos 78b-79a) elaborate on the actions of the Givonim, who took brutal revenge on the sons of Shaul because Shaul had indirectly caused them to lose their source of bread and water. Due to the brutality of the Givonim, Chazal decreed that since only one who exhibits chessed is worthy to marry into Bnai Yisroel, one may not marry a Givoni. The Givonim, who had suffered by having their own source of bread and water cut off, should have become more sensitive to others. Since they became completely insensitive, they were no longer considered eligible marriage partners.
Amon and Moav similarly were rejected for marriage because of a complete lack of middas hachessed. As descendants of Lot, they should have followed his example of hachnasas orchim and run to provide bread and water for Bnai Yisroel. Chessed is so essential to a Jewish marriage that one who lacks it cannot possibly be eligible to marry into kahal Hashem.
Although the Torah only gives two explicit reasons for the prohibition to marry descendants of Amon and Moav, Rashi (Devarim 23:5) sees a third reason alluded to by the phrase "al dvar," the attempt to seduce Bnai Yisroel into acts of immorality, as occurred with the women of Moav (Bamidbar 25). The Moavim couldn't bring bread and water to hungry travelers, yet they were able to invite others to their parties of idolatry and immorality.
It is not a coincidence that those who lacked kindness were steeped in immorality. In Vayikrah 20:17, an act of immorality is referred to as chessed. Chessed can mean either kindness or a prohibited incestual relationship because they both stem from the same desire to love and be close to others. Gemilus chassadim is an appropriate expression of this desire, whereas gilui arayos is a distorted expression. One who does not perform chessed may end up showing love in the most inappropriate manner. Amon and Moav, who rejected chessed as a way of life, became synonymous with immorality.
The failure of Amon and Moav to provide Bnai Yisroel with food and water indicated a fundamental flaw in these nations on two levels. The lack of consideration for others in and of itself was problematic; combined with the immorality that resulted from it, it rendered Amon and Moav ineligible to marry into Bnai Yisroel.
