On a deeper level, the four species are often compared to four types of Jews.
The etrog has taste and smell, so it stands of people who possess both learning and good deeds.
The palm tree has taste but no smell, so it stands for those with learning, but no good deeds.
The myrtle has smell but no taste, so it stands for people who have good deeds but no learning.
The willow, has neither taste nor smell, and it stands for those without learning and without good deeds.
Another view sees the lulav as one person. The lulav represents the spine of a person; the myrtle the eyes; the willow the mouth; and the etrog the heart. Through them, we express our desire to praise and worship G-d with our entire being.
Another theory is that the lulav and etrog symbolize roots, such as land and living, fruit and fertility, while the Sukkah is a symbol for the temporary, exile and wandering.