Commentary for Leviticus 19:18

לא תקם THOU SHALT NOT AVENGE — If one says to another “Lend me your sickle", and he replies, “No!", and the next day he (the latter) says to him (the former), “Lend me your hatchet”, and he retorts, “I am not going to lend it to you, just as you refused to lend me your sickle״ — this is avenging. And what is “bearing a grudge”? If one says to another, “Lend me your hatchet”, and he replies “No!” and on the next day he says to him “Lend me your sickle”, and he replies: “Here it is; I am not like you, because you would not lend me” — this is called “bearing a grudge (נטירה)” because he retains (נוטר) enmity in his heart although he does not actually avenge himself (Sifra, Kedoshim, Chapter 4 10-11; Yoma 23a).

There follows a general, all inclusive rule to be observed in relations towards one’s fellow, phrased as ואהבת לרעך כמוך, telling us to apply the same yardstick to our concern for our fellow that we would want applied to ourselves if we were in his shoes in similar situations.

לא תקום ולא תטור, "Do not take vengeance and do not bear a grudge." Yuma 23 defines vengeance as someone responding in kind to a hateful act by his neighbour he has experienced, whereas the "grudge" is defined as repaying a hateful act with a kind act pointing out, however, that one is morally superior to the person who committed the hateful act. The proof for this is the verse following that one should "love your neighbour as yourself." The reason the Torah has to say this is to indicate that it is G'd's wish that we relate to our fellow Jews with the same love we have for ourselves. If the person described as bearing a grudge told his neighbour that despite the fact that the latter refused to lend him his spade, he in turn was willing to lend him his own spade, he indicated that he had harboured resentment against his neighbour first. G'd tells us that the reason we must not bear a grudge is because the Lord is our G'd. This means that by means of individual Israelites experiencing a unification of their hearts, G'd's Unity itself is enhanced. This is all based on the kabbalistic concept that all Jewish souls are branches of the Holy name of G'd (י־ה־ו־ה) based on Deut. 32,9 "for His people are part of Him" (compare Zohar volume 3 page 16).