What advantage does the lulav possess over the esrog? Seemingly, it represents a lower level, for the lulav possesses only a pleasant flavor (the virtue of Torah study), while the esrog possesses both flavor and fragrance (both Torah study & the observance of mitzvos).
When a Jew contemplates the Torah, he is grasping G-d's wisdom, as it were, and his mind becomes one with G-d's wisdom; this highlights the advantage of the lulav -- those individuals who are totally devoted to Torah study, even at the expense of good deeds.
Each one of the four species reflects bittul, and this makes unity with the other species possible.
We must all devote ourselves solely to the Torah, in a manner which reflects the lulav; even a "willow" Jew must at certain times reflect the behavior of a lulav and study Torah in a complete bond of unity.
