Musar for Shabbat 302:12

And there is yet another reason for this. It is known that in accordance with a man's ordering of his traits in this world is the corresponding arousal of this trait in the world above. If it is his way to overlook offenses and to deal with men with the traits of lovingkindness and mercy, he thereby arouses, above, the trait of mercy, and the Holy One Blessed be He is merciful to the world because of him. And "from the fruits of a man's mouth, his soul will be sated." And he, too, will merit the mercies of the Holy One Blessed be He, and his offenses will be forgiven, as Chazal have said (Shabbath 151b): "All who are merciful to men are granted the mercies of Heaven," and (Yoma 23a): "All who overlook offenses to themselves have their [own] offenses overlooked [by Heaven.]" As stated in the Zohar, Parshath Emor:

And you, son of man, examine yourself and be far from cruelty and have mercy upon the poor and the needy and let the poor be as the members of your family. "Then the Lord … will show you mercy and have compassion upon you and multiply you" (Deut. 13:18). Everyone who has mercy on his fellow creatures will have mercy shown him from heaven (Shabbath 151b). Therefore, do not remove mercy from you but guard yourself against the quality of cruelty. And Solomon said: "Lest you give to others your vigour and your years unto the cruel (to being cruel)" (Prov. 5:9). And guard yourself against the desire for vengeance which itself stems from cruelty. If you wish to truly avenge yourself on your enemy then add to your good qualities and walk in the path of the upright, and in this way you will be avenged on him who hates you, for he will writhe in pain because of your good qualities and good repute and mourn when he hears good of you. But if you do ugly deeds then your enemy will rejoice over your shame and disgrace and thus avenge himself against you.

One who shuts his eyes from an opportunity to give charity, it is as though he served idols (Kethuboth 68a). Such a man is called "worthless," "wicked," "merciless," and a "sinner." Worthless, as it is said, "Beware that there be not a base thought in thy heart, saying : 'The seventh year, the year of release, is at hand;' and thine eye be evil against thy needy brother, and thou give him nought" (Deut. 15:9). Merciless as it is written, "But the tender mercies of the wicked are cruel" (Prov. 12:10). And he denies his lineage; he is not of the seed of Abraham, Isaac and Jacob, for they were merciful, but he is of the seed of the heathens who are cruel, as it is said, "They are cruel and have no compassion" (Jer. 50:42). And everyone who shows mercy, Heaven has mercy upon him (Shabbath 151b). As it is said, "… and show thee mercy, and have compassion upon thee, and multiply thee" (Deut. 13:18).