Since the first sin man commited by eating the fruit of the 'Tree of Knowledge Good and Bad', he and all creation became a mixture of good and evil. Only at the end of time everything will revert back to its original goodness, as we see in the beginning verses of our Parsha that differentiate between the positive that will happen (Asher Tishma'un) and the negative that might happen (Im Lo Tishma'un).
In the meanwhile, it is our job to seperate the Tree of Knowledge from its pure counterpart, the Tree of Life, by defining and limiting each to its boundaries. This process is symbolized by the two hills designated in the beginning of the Parsha, Grizzim and Eival, one for blessing, and one on which the negative forces should dwell and not spread to the rest of the Holy Land. In fact, the whole first half of the Parsha speaks about limitation of negative forces, while the second half speaks about freedom.
When a person takes a walk and sees a beautiful spot of nature and desires to bring a sacrifice to the Creator under a lush tree, he is forbidden to do so. Without restrictions of time and space the positive forces generated by his pure motives get sidetracked and lost. When a person is living comfortably and wants to enjoy beef, he is welcome to do so, but only after stringent measures are taken in its preparation: Shechita and removal of all forbidden parts. The blood still may not be eaten because the forces it contains are too powerful for us to elevate by eating. Everything must be clearly defined, especially truth and non-truth; animals that haved mixed signs of purity and impurity are specifically forbidden.
When it comes to the Shemita year, however, everything changes. On the Sabbatical Year there is no frugality. Within this realm where spirituality reigns there are no prescribed amounts to give your slave when you set him free or to bring Hashem on the holidays. Where limitation doesn't reach, there are no limitations.
Adapted from Rabbi Vali's book on Dvarim. Questions and subscriptions can be mailed to: the Yeshiva