Rabbi Hershel Schachter
Visiting, and Hosting, Hashem
The requirement to build a Beis Ha'mikdash is counted among the six hundred and thirteen mitzvos, and applies in every generation whenever possible. Yechezkel hanavi (11:16) told Bnei Yisroel that they would be exiled to Bavel, and even though they will not have the real Beis Ha'mikdash they will have a "mikdosh me'at - miniature Beis Ha'mikdash." The gemarah (Megillah 29b) understands this to be a reference to all shuls and yeshivos in Bavel and all over the world. The Chayei Adam (quoting R' Eliezer Mi'mitz in Sefer Yerayim) says that kidushas beis ha'kneses and Beis Ha'mikdash are of biblical origin. Rabbi Dovid of Novardok (Teshuvos Galya Masechta) assumes that according to the Ramban who writes (Parshas Naso) that there is a biblical mitzvah to celebrate upon the completion of the building of the Beis Ha'mikdash, it would constitute a biblical mitzvah to celebrate a chanukas ha'bayis upon the inauguration of a new shul or a new beis ha'medrash.
Rav Soloveitchik (Shiurim Lezeicher Avi Mori, Vol. 2, pages 78-83) explained that one of the main purposes of the Beis Ha'mikdash is for us to have a location where we can get together with Hashem. The Beis Ha'mikdash is beis Hashem, Hashem's home, and we go there to "visit" Him. Shuls and yeshivos, however, are our home and Hashem "comes" to them in order to "visit" us. When one goes to visit the King in his palace, one must be much more respectful than when the king is visiting in one's home. This is why the chumash speaks of morah ha'mikdash (extreme respect for the Beis Ha'mikdash) and the gemarah and Shulchan Aruch speak of k'vod beis ha'kneses, a slightly lower level of respect.
The Shulchan Aruch, quoting the Talmud Yerushalmi, says that bigdei yom tov must be fancier than bigdei shabbos. Rav Soloveitchik (ibid) explained that on the Shalosh Regolim we have an obligation to be oleh leregel, to visit the Ribbono Shel Olam in His palace. On Shabbos, on the other hand, the Shechinah comes to visit us. When we recite Kabbolas Shabbos, we are not only greeting the Shabbos Queen, but also the Shechina who is visiting us. Consequently the minhag in Europe used to be that everyone stood for Lecho Dodi; it was treated as a dovor shebikedusha, since the kohol was greeting the Shechina who was coming to visit us on Shabbos. Therefore, the bigdei yom tov should be more elegant than bigdei Shabbos because on the regolim we are visiting the King in his palace, as opposed to Shabbos when the King is coming to visit us.