The way in which one acts towards others determines the way in which Heaven will act towards him. For example, if one strives to overlook the hurt caused him, and if he acts towards others with kindness and compassion, then he awakens the Attribute of Compassion in Heaven.  Hashem will judge him with mercy and he will be shown kindness from Above.  As our Sages state: “Whoever has compassion towards others will be shown compassion from Above” (Shabbos 151b); and “Whoever refrains from exacting his measure [of retribution from others for the wrong they caused him] will have all his sins forgiven” (Rosh Hashanah 17a).  Moreover, in his merit, Divine compassion will manifest itself upon the Jewish nation as a whole.

However, if it is one’s way to be absolutely unrelenting towards others and to treat them without compassion, then he lends strength to the Attribute of Justice —toward himself specifically, and to the entire Jewish nation as well.  Heaven will be unrelenting regarding that person’s sins, for “With the measure that a person measures do they [i.e. Heaven] measure him” (Megillah 12b). ~ Chofetz Chaim