It states (19:18) תקום לא, "You shall not take revenge..." Sefer HaChinuch (241) writes, "The ways of most people is that if someone wronged them, they don't cease seeking opportunities to get back at them to give them what they deserve... Hashem yisbarach forbids us to do so. The Sifra gives an example of revenge that is forbidden. You need a hoe, so you ask your neighbor to lend you one, who refuses your request. Sometime later, your neighbor comes to you and asks for an ax. It is forbidden to reply, 'I won't lend you the ax, just as you didn't lend me the hoe.'
One of the reasons for this mitzvah is that a person should know that whatever happens to him, whether good or bad, is from Hashem yisbarach. It is impossible for a human being to harm his fellow man [if it wasn't decreed from Above]. And therefore, when someone harms you, know in your soul that Hashem is punishing you for your aveiros. Don't seek revenge. He isn't the cause of your tzaros.
[When Shimi cursed Dovid HaMelech], Dovid haMelech a”h said (Shmuel 2, 16:11), הניחו 'ה לו אמר כי ויקלל לו, 'Let him curse; for Hashem has told him [to do so].' Dovid attributed it to his aveiros, not to Shimi ben Gera.
