The Gemara states that an esrog is described in the Torah as a pri eitz hadar, the fruit of the hadartree, and the word hadar can be interpreted to mean haddar, that dwells. Thus, the esrog is a fruit that dwells on its tree from one year to the next year.
It is noteworthy that the Rishonim write that the word esrog is derived from the Aramaic word merogeg, which means desire. Similarly, we find that the Zohar states that regarding Shabbos it is said the Children of <st1:country-region w:st=”on”><st1:place w:st=”on”>Israel</st1:country-region> shall observe the Shabbos, to make the Shabbos an eternal covenant for their generations. The Zohar states that the word ledorosam can be interpreted to read lederosam, for their dwelling places. Furthermore, it is said thus the heaven and the earth were finished, and all their array, and the Targum Yerushalmi interprets the word vayechulu, were finished, as vechamad, and He desired. Thus, the esrog and the Shabbos both share the same characteristics in that they are desired items and that they both have a permanent dwelling.