Rashi asks what connection there is between the place and the message. He tells us that just as the sabbatical year was proclaimed at Sinai, so were all the commandments.
If we look for an ideological link between Sh’mittah and Sinai, it may be that Shabbat figures prominently in the Decalogue given at Sinai, and an extension of the weekly Shabbat is the 7th-year Sh’mittah.
If we look further, we find that taking off one day in seven and devoting it to God’s purposes, and devoting one year in seven to the Almighty, tell us that ultimately everything we have and do is due to the Creator. The person who enjoys blessings from life owes his/her good fortune to God. The person who suffers from problems not only knows that in God he/she has someone to argue against, but since evil as well as good emanates from the same Creator there must be some purpose in whatever befalls a person.
We are probably too small in the scheme of things to understand the details of God’s designs, but there is a comfort in knowing that God is in charge.