Why do we keep mitzvos? A good question, asked by all of us, I presume, at one time or another. Three of our favorite commentators see defenses for their own pet theories in a passage from this week's parsha.
Exodus 13:5-7: It shall be when the LORD brings you to the land of the Canaanite, the Hittite, the Amorite, the Hivite and the Jebusite, which He swore to your fathers to give you, a land flowing with milk and honey, that you shall observe this [Passover] rite in this month. “For seven days you shall eat unleavened bread, and on the seventh day there shall be a feast to the LORD. “Unleavened bread shall be eaten throughout the seven days; and nothing leavened shall be seen among you, nor shall any leaven be seen among you in all your borders.
In Hebrew, the next sentence (13:8) says:
והגדת לבנך ביום ההוא לאמר בעבור זה עשה יהוה לי בצאתי ממצרים
What does this mean? Well, it depends who you ask. The difficulty is the word זֶה (which means this or that), a pronoun with an antecedent that isn't immediately obvious.
According to Rashi, זֶה refers to mitzvos (for instance those detailed in the preceding verses) so the last line should be translated as follows: And you shall tell you children on that day, saying: For the sake of this (i.e. the mitzvos) the LORD took me out of Egypt. Thus, according to Rashi, mitzvos are an ends unto themselves, and the LORD rescued us from Egyptian servitude so that we might perform them.
The ibn Ezra gives (more or less) the same reading and presents (more or less) the same theology.
The Ramban disagrees. As he sees it, זֶה refers to the miracles the LORD performed at the Exodus, thus: And you should tell your children on that day, saying: For the sake of this (i.e. the wonders and signs that accompanied the Exodus) which the LORD did when he took me out of Egypt... meaning: we do the mitzvos (for instance those detailed in the preceding verses)for the sake of the wonders and signs that accompanied the Exodus.
According to the Ramban, therefore, the commandments are performed for the purpose of telling ourselves and others that God exists, runs the world, rescued us from Egypt, etc. Thus commandments are a means to an end, and not an end in of themselves. The goal is to retain and/or publicize the idea represented by the commandment, not to keep the commandment for its own sake.
The Ramban has a great deal more to say on this subject and supports his POV with a second, sod, translation of the verse, in which he says the word זֶה additionally represents the majesty and glory of God, thus For the sake of this (i.e. His own glory) the LORD took me out of Egypt...
Commandments, then according to Ramban, are kept as an ongoing testimony and a never-ending attempt to publicize God's glory/existence/majesty/providence, etc.