Hashem Gives to Him so That He Will Be Able to Give to Others

Tzedakah is a unique mitzvah, for one emulates Hashem by providing for others. When someone makes this his goal, Hashem helps him from on High. Anyone who bestows good on others is like a flowing wellspring. Just as a wellspring of water is constantly replenished from its source, Hashem, Who is the Source of all tzedakah, constantly bestows and replenishes those who choose to be wellsprings of giving. This is a deep concept.

(Maharal on Agados Hashas)