Why do we have to give Tzdaka? Does Hashem need our help? Can't HE help out the poor guy? Maybe if the guy is poor Hashem wants him that way and we shouldn't get involved?! Why are Rabbis always preaching to me about how I should give Tzdaka?? They should mind their own business and not tell me how to spend MY hard earned money!!
We NEED to give primarily b/c our souls NEED to do good. Our body is nourished by food while one of the greatest sources of nourishment of SOUL is KINDESS. A Goy [and many Jews who don't know any better] give b/c they feel badly for the poor guy. צדקה תרומם גוי וחסד לאומים חטאת - משלי יד לד. The chesed of the Gentiles is called "חטאת". The word "חטא" means lacking:
כמו שאומרת בת שבע לדוד: "והיה כִּשְׁכַב אֲדֹנִי המלך עִם אֲבֹתָיו (ואדֹניהו ימלוך תחתיו), וְהָיִיתִי אֲנִי וּבְנִי שְׁלֹמֹה חַטָּאִים" (מל"א א, כא). [אם אדוניהו יתפוס את המלכות, תחסַר המלכות ממני ומבְּנִי].
They give b/c the poor person lacks. That is LOVELY but not the highest level of giving.
A Jew should give b/c it is the proper thing to do. It makes one holy and elevated. צדקה תרמום גוי. Tzdaka ELEVATES the nation [of Israel].
The Medrash connects נעשה ונשמע to giving Tzdaka "ויקחו לי תרומה". When we said נעשה ונשמע, we went above any חכמה or חשבונות. We will do b/c it is right and not b/c we necessarily understand why. We will also GIVE b/c it is right and not b/c of how we understand the benefit of our giving. 
