In this weeks parsha we find the well known halacha, "a Moavi or an Ammoni shall not enter the Nation of Hashem forever. Because of the fact that they did greet you with bread and water... and because they hired Bilam... to curse you." The Ramban explains that this prohibition is a consequence of the lack of gratitude they displayed to the descendants of Avraham, to whom they owe their very existence. It was in Avraham's merit that their forefather Lot and his daughters had been saved from the destruction of Sedom.

The above is troubling for two reasons. Firstly, because your great-great-grandfather did a favor for my great-great-grandfather, I am eternally expected to reciprocate? Secondly, why would this failure result in such harsh repercussions?

To understand this we must first define the mitzvah of hakaras hatov, recognizing the good done for you. Rav Yitzchak Hutner, zt"l (PachadYitzchakRoshHashanahMa'amarGimmel) explains that when examining the laws governing hakaras hatov one realizes the depth of this mitzvah. There is a law which states that if one finds two lost objects, one belonging to his father and the other to his rebbe, and he is only capable of returning one of the objects, he should return that of his rebbe. The reason is that the honor of one's rebbe supersedes that of his father, "for your father brought you into this world, whereas your rebbe brings you into the next." [This changes if one's father hired the rebbe. In that case he channels his greater hakaras hatov to his father and therefore his father's lost object takes precedence.] What we learn from the above is that hakarashatov is not just reciprocation of the good done for you; it is a mitzvah with halachic ramifications.

Another angle in understanding hakaras hatov is revealed to us from a Gemarah in BavaMetzia. Our sages tell us that all the kindness that Avraham did for his guests and the manner which this chesed was performed was reciprocated by Hashem to the Jewish people in the desert. Those acts that Avraham performed himself, Hashem performed for B'nei Yisroel Himself. What Avraham did through a messenger, Hashem, too, brought it about through a messenger. The Maharal asks why only in regard to the mitzvah of inviting guests do we find Hashem repaying it in this parallel manner? The Maharal answers that gemiluschasadim, benevolence, is unique amongst the mitzvos, for it bears fruit even in this world. The seed of an apple, when planted, results in more apples. Similarly with gemiluschasadim, when one does good for another he is planting seeds of kindness in his friend which sprout forth similar acts of kindness in return. This does not extend only to his friend but even to the following generations, for the tree keeps on bearing fruit. Rav Hutner continues that one can understand from this that if one is unkind to someone by causing pain or damage, it is terrible in and of itself; but if one takes taking that seed of kindness that was planted inside him by his friend and does not reciprocate that good, thereby ruining that seed, that is "achzarious," cruelty. For he is not only destroying the present he is destroying the future.

Based on the above we can answer our two questions. The reason one has to reciprocate the good done to one's forefather is because chesed is not just a favor; it is a seed planted with ramifications for years to come. Just as Avraham performed chesed in this world and Hashem did chesed in return for his descendants, so does one have to reciprocate the good even to subsequent generations. The reason the Ammoni and Moavi can't join the Jewish people, is because of the cruelty they displayed as nation, taking the seed of kindness planted years before and destroying it. A nation who is cruel by nature cannot join a nation who is by nature merciful, for the chemistry is not compatible. May the seed of kindness implanted in our souls by our forefather Avraham flourish for us and generations to come.