Takana Considerations

Rebbe Yochanon Ben Zakkai led the Jewish nation through the tumultuous years immediately following the destruction of the Second Beis Hamikdash. His guidance preserved Torah-true Judaism. He made a number of decrees that became necessary after the Churban.
A Mishna in Rosh Hashana (30a) recounts two of these decrees. In the times of the Beis Hamikdash, everyone fulfilled the mitzva of the Arba Minim for only one day. The exception was those that visited the Beis Hamikdash. In the Beis Hamikdash, the Arba Minim were taken for seven days. After the destruction of the Beis Hamikdash, everyone without exception would have fulfilled the mitzvah for only one day. Rebbe Yochanon Ben Zakai saw fit to enact a reminder of the happier times and instituted that henceforth everyone should take the lulav and esrog for seven days (except for Shabbos).
Rebbe Shlomo Zalman, zt”l, therefore said that it is important for all to remember why we take the lulav for the entire Sukkos. It is to recall the way things were in the Beis Hamikdash. Indeed, he used to remind his own family of this on Sukkos. If one doesn’t recall the reason, he hasn’t fulfilled the zecher l’Beis Hamikdash properly.