I was most inspired by the teaching of the Kli Yakar on the second verse of parshas Acharei Mos wherein the Torah teaches that the Kohen Gadol "shall not come at all times into the sanctuary". He notes that it is not only that the High Priest cannot come on a regular basis, but he is to enter the Holy of Holies only once a year on Yom Kippur. He suggests the following insightful explanation: the effectiveness of the Kohen Gadol atoning for the sins of the Jewish people is dependent on the worthiness of the people. All year long the nation unfortunately is engaged in various sins and negativity which creates a separation between themselves and Hashem. This state of sin creates a barrier between Bnei Yisroel and Hashem, and since the effectiveness of the Kohen Gadol is dependent on the people, all year long he cannot truly represent them. However, one day a year, on Yom Kippur, when the entirety of the nation is abstaining from food and all this worldly pleasures and compared to angels, then and only then can the Kohen Gadol represent the Jewish nation before Hashem as they empower and create a positive and holy energy to enable the Kohen Gadol to effectively atone for the nation of Israel.
The Torah teaches (Shemos 20:8), "remember the Shabbos day to sanctify it" and we therefore take a cup of wine and recite the blessing of Kiddush every Friday night. If Hashem already sanctified Shabbos, as is stated in the paragraph of Vayichulu, (Breishis 2:3), "Hashem blessed the seventh day and sanctified it", what are we accomplishing with our sanctification during Kiddush? The Ramban explains that just as when the Torah teaches (Vayikra 25:10), "that you shall sanctify the fiftieth year" it is understood by our Rabbis to mean that the Sanhedrin is to formally sanctify the Jubilee year, so too every Jew is called upon to add his sanctification of the Shabbos day each week as well.
Similarly, regarding the law of bechor biheima tahora (Devarim 15:19), "the first born male that is born in your cattle and in your flock, you shall sanctify to Hashem", the Talmud Yerushalmi (Rosh Hashanah 1:1) teaches that the first born is holy automatically at birth. And yet the Talmud (Erchin 29:A) teaches that even though it is endowed with holiness from birth, there is still a mitzvah to articulate and announce that this animal has the sanctity of a first born. Once again, we see that man is invited to participate and add sanctity to that which is already in a state of kedusha.
A Jewish mother endows her baby boys with their Jewish identity. Even so, on the occasion of the son's bris milah, the mohel announces "bris kodesh", stating that he is about to add additional holiness to the child through the mitzvah of circumcision. When the child has been circumcised, man has endowed the child with greater holiness and enabled this child to enter the Beis Hamikdash and eat kodshim.
Rashi (Breishis 1:1) teaches that the account of creation is the Jewish nation's deed to the Land of Israel. Hashem, who created the world, chose the Land of Israel for His nation from the very beginning of time. The Torah teaches (Devarim 32:8-9) that Hashem gave each of the nations their land/inheritance, and He gave His portion/land to His people, the descendants of Yaakov. There are many verses in the Torah which describe Hashem's giving the Land of Israel to the Jewish nation even before there were any descendants of Avraham (as found in Breishis 15:18). However, I'd like to demonstrate that here too, regarding the sanctity of the Land of Israel, we find that when Joshua brought the nation of Israel into the Land of Israel, most of the agricultural laws (with the exception of challah) that apply only in the Land of Israel were not applicable untill after the fourteen years of kibush - conquering and chaluka - dividing the land. Only then were the agricultural mitzvos binding upon the people. Once again, we see that while Hashem sanctified the Land of Israel, man contributed by his participation in endowing the land with further holiness.
For almost two thousand years, the Land of Israel has displayed its eternal bond with the Jewish people. On the one hand, we have been privileged to see the fulfillment of the prophecy that Hashem promised the Jewish people amidst the tochachah (Vayikra 27:32), "I will make the land desolate and your enemies who will dwell upon it will be desolate". Hashem kept His word and throughout the long exile that the Jewish people were completely separated from the land of Israel, the land was kept in the state of shemama - desolation. The land would not produce on behalf of our enemies. On the other hand, with the return of the Jewish nation to its land we have seen the fulfillment of prophecy of Ezekiel (36:8), "Your mountains of Israel your trees will bear branches leaden with fruits for my people Israel as they return". We have seen the sense of loyalty on the part of the land - she has never betrayed her people, has never given herself to strangers and conquerors, neither Rome, Byzantium, Moslems, Crusaders, Turks, or British; they all failed in colonizing the land. And yet now, Israel continues to blossom and shine agriculturally, industrially and scientifically with advanced technology. Most of all, we have witnessed the fulfillment of (Isaiah 2:3 ), "Ki MiTzion Taytzay Torah". The incredible amount of Torah study and proliferation of Torah seforim has been a crowning achievement of Israel over these past seventy five years.
The Ramban (Vaykira 18:25) cites the Sifrei (Devarim 11:17) regarding the juxtaposition of Hashem's warning the Jewish nation that if they fail to adhere to the Torah and mitzvos they will be expelled from the good land that Hashem has given them, and the Torah teaching,"You shall place these words of Mine on your heart and upon your soul, you shall bind them for a sign on your arm and let them be an ornament between your eyes. You shall teach Torah to your children, and you shall write them on the doorposts of your house and upon your gates". This juxtaposition of being expelled from the land and the performance of mitzvos is to teach that while the Jewish people are exiled from their land they shall still fulfill mitzvos so that when they return, mitzvos will not be a new phenomena since they had been observed and practiced all the years. The Ramban further states unequivocally that the primary location for the performance of all mitzvos is the Land of Israel.
There is a difference of opinion between the great Rabbinic commentaries regarding the construction of the third Beis Hamikdash. Rashi (Rosh Hashanah 30a) explains that the third Beis Hamikdash has already been constructed by G-d and exists in the heavenly realms waiting for the time that it will descend to the earth. The Rambam (Mishneh Torah, Hilchos Melachim 11:1) is of the opinion that the third Beis Hamikdash will be built by man with mashiach overseeing its construction. While time will tell which of these two giants are correct, one thing is clear: that without the Beis Hamikdash we are in a state of galus, meaning that we are not yet worthy of having the Shechina in our midst, as we recite in the musaf of Rosh Chodesh and the Shalosh Regalim, "Because of our sins we have been exiled from our land and lost the mikdash, the privilege of having His Shechina in our midst". Thus, observance of mitzvos in the Land of Israel is, in reality, a double observance. It is the observance of the mitzvos per se (Shabbos, kashrus, etc.), and in addition, we are creating and adding kedusha to the land, which is a precursor to the arrival of the third Beis Hamikdash.
Finally, the Talmud (Kesubos 75a) cites the verse (Tehillim 87:5), "And to Zion it shall be said: This man, this man was born in her." Rashi explains that the verse describes the future when the nations of the world will return Jews to Tzion saying, "this one is the son of Tzion, he was born there, let us bring him back to her." The Gemara explains that this refers both to those who were actually born in Tzion and to those who yearn to see her; both those who were actually born in the Land of Israel as well as those who yearned for the return to Tzion will be counted as her children. Our personal goal and aspiration should be to live there and personally add to the holiness of the land. However, if that is not possible, one's support of Torah and enhancement of the quality of life in Israel certainly adds kedusha to the Land of Israel.