One of the major topics in the Torah is the subject of ownership, how to determine who owns what. Why is this so important? Why is the legal system of ownership the focus?
Rabbi Mattisyahu Salomon answers this question in his shiur on kindness. He tells us that in order to live, the body needs to take. But for the soul to live and flourish, the soul needs to give. This is quite a contrast, a duality for us, because we are souls within bodies. The importance of ownership is that, if we do not own something, it is impossible for us to give it to another person. Without knowing who owns what, there is no way for the soul to give and thereby grow.
This is the key concept behind charity - we give because it is good for the giver to do so. When we are able to give to another person, we emulate the Creator Who is constantly giving to us.
Why is giving and emulating the Creator important to our spiritual growth?
Rabbi Ezriel Tauber, who survived World War II as a child, explains that the reward for our lives is eternal connection to the Creator. He explains that if we have not ingrained upon our souls the joy of giving in this world, it will not feel like a reward for all eternity. Imagine! If a person gets a whole gallon of ice cream and can't taste it, it is no fun at all to eat. That is why we develop a taste for giving here...so that the reward in the next world will feel like a reward.
Here is a book by Rabbi Tauber: Thoughts for a Jewish Heart
The subject of charity invariably deals with kindness.