Our Gemara discusses the prohibition of revenge and grudge bearing:
What is revenge and what is bearing a grudge? Revenge is illustrated by the following example: One said to his fellow: “Lend me your sickle”, and he said: “No”. The next day he, the one who had refused to lend the sickle, said to the other person: “Lend me your ax.” If he said to him: “I will not lend to you, just as you did not lend to me, that is revenge.”
And what is bearing a grudge? If one said to his fellow: “Lend me your ax”, and he said: “No”, and the next day he, the one who had refused to lend the ax, said to the other man: “Lend me your robe”; if the first one said to him: Here it is, as I am not like you, who would not lend to me, that is bearing a grudge. Although he does not respond to his friend’s inconsiderate behavior in kind, he still makes it known to his friend that he resents his inconsiderate behavior.
The Gemara eventually draws a distinction between “financial pain” and “personal pain”, that is the prohibitions of revenge and grudges would seem to be limited to withholding or resenting the withholding of goods and services, not the resentments or reaction to anguish from interpersonal quarrels. Thus, if a person is resentful that his or her friend did not lend him an object, to refuse to return a favor stemming from this resentment in the future is forbidden revenge (lo sikom), and even to harbor the feelings inside (see chinuch 242) would be forbidden as grudge-bearing (lo sitor). However, if someone hurt your feelings or said something mean to you, harboring a grudge would not be forbidden, and confronting the person so long as you were not hurtful would not be forbidden. Incidentally, The Chofets Chaim (באר מיים חיים (ח-ט) הקדמה להלכות לשון הרע) cites an opinion that to fire insults back at the moment that someone else is insults you is considered self-defense and not necessarily under the prohibition of hurtful speech, though it still is an extra pious act to show forbearance. Additionally, The Chofets Chaim (ibid) cites a number of opinions which hold that all the prohibitions apply even for matters of emotionally distressing anguish and strife, despite the way the Gemara seems to imply. Those poskim hold that the verbal vindictiveness mentioned in the Gemara was only allowed for a Sage to protect the honor of the Torah, but not for regular citizens. Regardless, the Chofets Chaim maintains that even according to the opinion that the prohibition is limited to financial pain, it is not only literally about money. Any withholding of a good, service or kind act is included in the restriction.
Now that we have analyzed the Talmudic portion, let us look at the original set of verses, to understand how the Torah is offering guidance to promote and preserve harmony in relationships. These verses (Vayikra 19:17-18) have their own separate derashos, but in plain text meaning, tell one consistent story:
לֹֽא־תִשְׂנָ֥א אֶת־אָחִ֖יךָ בִּלְבָבֶ֑ךָ הוֹכֵ֤חַ תּוֹכִ֙יחַ֙ אֶת־עֲמִיתֶ֔ךָ וְלֹא־תִשָּׂ֥א עָלָ֖יו חֵֽטְא׃
You shall not hate your kinsfolk in your heart. Reprove your kinsman but incur no guilt because of him.
לֹֽא־תִקֹּ֤ם וְלֹֽא־תִטֹּר֙ אֶת־בְּנֵ֣י עַמֶּ֔ךָ וְאָֽהַבְתָּ֥ לְרֵעֲךָ֖ כָּמ֑וֹךָ אֲנִ֖י יְהוָֽה׃
You shall not take vengeance or bear a grudge against your countrymen. Love your fellow as yourself: I am the LORD.
The verses instruct a person not to hold anger and hate in the heart. Instead, one should openly speak out what is the source of insult or resentment in order that no further sin is incurred, presumably by either party. Therefore one should not bear grudges nor take revenge, allowing for loving thy neighbor as thyself.
In terms of relationship harmony, the directive is that one Is forbidden to hold angry thoughts inside. Instead, in a respectful manner one is supposed to explain to the other person how he or she was hurt. One is not allowed to let it fester. Maimonides (Deos 6:6) codifies these verses in the same simple pshat manner:
If one man commit a sin against another man, the one sinned against shall not remain in silent hate against the sinner, as it is said of the wicked: "And Absalom spoke unto Amnon neither good nor bad; for Absalom hated Amnon" (Second Samuel. 13.22); but, on the contrary, it is obligatory upon him to make known to him and say unto him: "Why have you done to me thus and such, and wherefore have you sinned against me in that particular matter?"; for, it is said: "And thou shalt indeed rebuke thy neighbor" (Lev. 19.17). And, if the sinner did repent and begged to be forgiven by him, he must forgive him; and in doing so he should not be cruel, for it is said: "And Abraham prayed unto God" (Gen. 20.17).
However, that is only part one. Part two is that even in action one must not retaliate. What is not allowed to withhold a kindness, good or service, in response to what someone else has done. What that means is, that if your spouse didn’t do something for you and you were angry at him or her, number one you must confront respectfully and not hold it inside. Number two, you are certainly not allowed to retaliate or withhold anything. Folks, that means possibly even a smile or a good morning, let alone more important loving acts and gestures.
This is a fabulous relationship technique because it also stops painful and hurtful situations from spiraling out of control. You know, often we don’t realize when we offend or hurt somebody else. We are far better at noticing when others hurt us. So, often in relationships, we are reacting angrily to the perceived hurts and actions that others do toward us. We rationalize, “he or she started“. In reality, that might be true or we might not realize that we actually were the offensive person first. We are unaware of the hurtful patterns of our speech, the condescension, the sarcasm, or who knows what else. In any case, since the Torah forbids us to retaliate and react by withholding a good or service and also requires us to respectfully confront, this nips the problem in the bud. And, if whatever reason, we aren’t being heard, it forbids us from escalating further.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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