Rapid Change
Rashi notes that the laws of human purity follow the laws of animal purity. Rabbi Simlai explains that just as man was created after the animals in the Six Days of Creation, the Torah records the laws of human purity after those of the domestic animals, wild beasts, and birds.
The Lekach Tov asks a fundamental question: Why are the laws of tumah (impurity) and taharah (purity) so different for humans?
Animals are fixed. However, a human being is unique; our spiritual status can change numerous times from tahor to tamah and back again. What is it about our nature that allows for such fluidity?
Even more puzzling is that the laws of tumah regarding humans are much stricter. If one touches one of the "eight swarming creatures" (shmoneh shrutzim) while it is alive, there is no transference of impurity. It must be dead. But a human being can transfer tumah even while he is alive.
Hashem did not originally create man with an aspect of tumah. The Nefesh Hachaim explains that impurity was only introduced to the human experience after Adam ate from the Etz HaDaat (Tree of Knowledge). Since that moment, it has become difficult to maintain 100% kedushah. Even when we strive to act purely for the sake of Heaven, our actions often remain a mixture of good and bad.
Animals are bound by their specific attributes; they remain what they are forever. We, however, were given Free Choice. Because of this, we fluctuate between holiness and impurity. Turning away from Hashem lowers our status, as we see in the laws of tzara'at later in the portion.
On the other hand, because we were created even after the angels, their celestial attributes are also woven into our souls. We are not bound by the material world. At any given moment, we can rise above the mundane forces that pull us down. We do not have to search far for this strength—it is part of who we are.