Rabban Gamliel, son of Rabbi Yehudah the Prince said:
Torah study goes well with work, since the toil of them both banishes misdeed.
-Pirkei Avos (Ethics of the Fathers), 2:2
The act of earning a living actualizes the goal of creation, which is to make the material world a Divine home. This can only be achieved by those who deal directly with the material world in a sanctified manner.
- Likkutei Sichot, 30:138
A person is made of two parts: the body and the soul. The body is there to serve the soul. In the same way the body is there as a conduit for the soul - our physical work is there as a way to sanctify G-d.
When we are honest in our business dealings, when we are polite to the clerk at the store, when we help others, when we do the mitzvot - these are all ways that we change our physical world into a spiritual one - a home for G-d.
Every Jewish person, whether they recognize it or not, has a mission to make the physical into the spiritual. The way we accomplish this is through the mitzvot. Why the mitzvot? Can't we be good people without being told what to do?
Of course we can be good people without the mitzvot. The catch is - that we don't always know what it means to be "good". But we'll save that discussion for a different day.