Kindness

We learn in the parsha (14:8) that it is a positive commandment to give charity to a fellow Jew.   The Torah stresses that one should give charity with simcha (ibid verse 10.)  Why is there an emphasis on simcha by giving charity more than any mitzvah?  Why is it that if one gives charity for a reward he is considered a great tzaddik (Rosh Hashana 4a), he gave for his own purpose; it’s not a perfect mitzvah?  Why is it specifically by charity that G-d promises to pay back what one spent and one is even allowed to test G-d to fulfill his promise (see Malachi chapter 3)?  Similarly, the Gemora tells us in Taanis (9a) that if one gives maaser one will get rich.  Why are these promises made specially for charity?
The answer lies in the Gemora Yevomos (79a) which describes the nature of Bnei Yisroel as גומלי חסדים.  Doing chesed isn't an action one does, but rather is an outgrowth of the nature of Bnei Yisroel (see Or Hatzofon chapter 7.)  Doing chesed is a din in the gavra, not in the cheftzah of giving.  That is why even if one has an alternative reason for giving charity it still is considered a great deed, for it arouses the natural desire one has to do good for someone else.  (See Rashi in Rosh Hashana along these lines for why it is a positive deed only when a Jew gives charity for reward but not for a gentile.)
Tosfos in Bava Metziah (70b) says that if one takes interest from a loan he does not merit to be revived when the dead come alive.  Why is there such a severe punishment for taking interest?  The Gemorah at the end of Sanhedrin defines a wicked man as one who steals.  Why is this the ultimate evil doer?  And conversely why is the tzaddik one who gives to others?  The explanation may be along the same lines.  If one doesn’t give charity it shows he has even corrupted the basic traits of a Jew and therefore doesn’t merit to be resurrected.  One who goes even further, to steal is the ultimate evil.
We may add another label of explanation.  The whole creation is merely a kindness of G-d, which means one’s whole existence depends upon charity. Therefore, built into the very existence of the whole creation is the need for charity. Based upon this idea, the Bais Halevi in his drashos # 1 explains why by this mitzvah one can test G-d, for charity is what sustains the whole world and when one attaches himself to this trait he merits that G-d will pay him back for upholding the world. Based upon this Beis Halevi we can explain the need for simcha in giving charity as well.  Since kindness is what keeps the world running, it behooves one to be in a state of simcha to recognize this fulfillment of the world.  When one is fulfilling his potential, one will feel happy as the Maharal often writes.  (See the Bais Halevi’s explanation about simcha in a different vein.)   The Ekarim volume 3 chapter 33 says that in fact the reward of charity only comes from the simcha infused into it. Now we can understand the severity of taking interest.  When one takes interest, it shows a rejection of the kindness of Hashem to give him life and therefore, the person doesn’t merit further life in the time of techias hamasim.