What causes a person to have a longing for Hashem and to become better? Rebbe Nachman teaches us that our longing comes as a result of us being held back. The greater the object of desire, the greater the obstacle to get it. When our desire is increased, our pursuit for the object is more powerful. The physical delight that touches our soul is through smell. Hashem is constantly sending us hints that we cannot see but we can smell from a distance. Eretz Yisrael (the land of Israel) is the headquarters of desire, therefore it has the most obstacles. The mistake of the spies was that the obstacles for Eretz Yisrael were so powerful that they thought it was greater than any desire to reach it.