If someone tells himself, “Why should I have mercy on someone else? He has nothing to do with me!” then he will be treated in the same way from Shamayim...but if he has mercy on others, then Hashem will respond to him in kind. Hashem hears the cry of the poor man, for He knows his pain. This is because the whole world is one entity, and every facet of creation is part of Hashem. One who chooses to see others as part of himself will merit to receive the same treatment from Hashem.
(Iyei Hayam)