And you shall take for yourself on the first day the fruit of a [citron] beauteous tree (Leviticus 23:40).
The halachah requires that an esrog must be beautiful, meaning that it must be free of blemishes. Even a minor defect may disqualify an esrog.
Why are the specifications for the esrog stricter than those for the other three species? Why is virtual perfection demanded only for the esrog?
The Midrash states that the leaf of the myrtle branch, is shaped like the eye, and its use in the mitzvah of the Four Species symbolizes to us that we must dedicate our eyes to the service of God, and not allow them to gaze upon things that would tempt us to sin. The leaf of the willow branch, resembles the lips, teaching us to guard our lips from speaking evil. The palm branch, represents the spinal cord, which controls all our actions, symbolizing that they are all dedicated to fulfilling the Divine will. The esrog resembles the heart, for one's thoughts and feelings should be absorbed with sanctity.
Ideally, while sight, speech, and deed should be completely involved with holiness, a deviation in any of these areas may be an isolated phenomenon and may not affect the whole being. Not so with thought and feeling. They affect everything one does. The heart's devotion must be complete, and there is, therefore, a greater requirement that the heart be pure.
The esrog, which represents the heart, must therefore be completely beautiful without the slightest defect.