The reason behind the Mitzvah of Shemitah:
The purpose of the Mitzvah of Shemitah is for one to be constantly reminded of creation and contemplate that Hashem created the world in 6 days. It publicizes to the Jewish people that everything belongs to Hashem and in the end will return to Him. It is a reminder of the future events that will occur, of the time where we will merit eternal rest [in the seventh millennium]. Alternatively it is in order for one to strengthen his Bitachon [trust] in Hashem. Alternatively it is to teach one the attribute of giving and compromise. Alternatively it is for the land to rest from its constant production in order for it to be able to continue to produce vegetation in the future years.
Sparks of Kabala
During the Shemitah year the Sefirah of Malchus is elevated, similar to the elevation that occurs on Shabbos. It is for this reason that it is forbidden to work the land during Shemitah, similar to the law on Shabbos. Furthermore, this is a cause for Simcha, joy, during this year. It is for this reason that the Tikkun of Rachel is not recited the entire year during Shemitah.
 Chinuch Mishpatim Mitzvah 69 “The purpose of this Mitzvah is for us to set in our hearts and impress within our minds the concept of the creation of the world that in six days Hashem created the Heavens and earth.”
 Chinuch Behar Mitzvah
 Ramban Behar “The days of the week hint to the creation of the world while the years of Shemitah hints to the future years of the world [that we will reach a state of Olam Haba, eternal Menucha]. One who denies it is like one who denies the creation”
 Keli Yakar Behar; Chinuch ibid “It also adds to one’s trust in Hashem”
The Keli Yakar ibid records the other reasons mentions by the Chinuch and Moreh Nevuchim and negates them for reasons mentioned there. He concludes that “I say that the reason for this Mitzvah is in order to root within the Jewish people the attribute of trust [Bitachon] and faith in Hashem . Hashem was worried that the constant preoccupation with the working of the land will cause them to forget G-d and they will think that it is their own efforts that has given them prosperity and blessing and that the world works based on nature. They will think that they own the land and they are the owners and no other. Therefore Hashem removed us completely from the natural order. This is a clearer and better reason than all the other explanations and it is for this reason that such a severe punishment of exile is meted out for one who transgresses, as it is a lack of faith in Hashem and a lack of belief in His ability to perform a miracle and provide them their sustenance. The earth itself will also be upset for it not being rested as it desired that through it the Jewish people add in their faith in G-d.”
 Chinuch ibid “It also helps one acquire the attribute of compromise and letting go”
 Moreh Nevuchim 3/39 “It is to benefit man that the land adds to its grain and strengthen its soil by resting”, brought in Keli Yakar ibid
 Kaf Hachaim 581/75