Answer: We need these desires to exist outside of the spiritual world. They give us the opportunity to ascend without being dependent on the spiritual world, without being bribed by it.
This allows us to live without “spiritual favors.” We continuously decide whether or not we should unite with our neighbors, the group. Sometimes we want to drop everything and leave, and we really do have that opportunity.
On the other hand, if we only had desires related to the group, we would be similar to, say, ants. Without having anything else, we would live the life of society involuntarily, and would again be on the animate, bodily level.
However, I live in my body and can aspire to spirituality or not aspire to it. This allows me to be a spiritual person because I reach the form of bestowal on my own, in spite of my natural, initial desire. The Creator made the evil inclination in me so that by overcoming it, I would become a free person. And I decide whether I want this or not by my own free choice.
Otherwise I would live in the spiritual world, but on the animate level. I would be an “angel”—a “spiritual animal” that instinctively loves everyone.
From the 4th part of the Daily Kabbalah Lesson 11/23/11, “Mutual Guarantee”