Prayer, I have argued with success, serves only to improve us, offering no benefit to God, and effecting no change in Him. Well what about mitzvos? Do I say the same about the commandments God has told us to keep? Certainly. The goal of every single mitzvah is to improve man. Not one of them adds to the glory of God or benefits Him in any way.
The Sefer Hachinuck makes an argument in this regard that I find compelling: (note I am not telling you this because I expect you to fall to your knees and accept it by virtue of the fact that the S.H said it. I want you to think about it, on it own terms. Pretend I said it, not the S.H, and please argue just as viciously) Why are we commanded to give charity? Couldn't God help the poor man on his own if he wanted? Couldn't he have created a world without poor people? The mitzvah, concludes the S.H, was not given to us for the sake of the poor, but like all mitzvos for our own sake alone. God wants us to develop our sense of compassion.
The Ramban (again, the name is provided for informational purposes only. I'm not expecting you to accept this argument simply because the Ramban made it) says something similar on the mitzvah of shiluach hakaan. God doesn't care about birds, and has no mercy for them. What he wants is for us to learn to distance ourselves from cruelty. "What difference does it make to the Holy One, blessed be He, whether an animal is slaughtered from the front of the neck or the back? Surely you must say the commandments have been given only for the purpose of refining men through them."
Disclaimer: The preceeding does not apply to every ritual act a Jew performs. Much of them aren't "mitzvot," but cultural practices we accumulated through our interactions with other people. I haven't supplied a way to determine which commandments are legitimate and which are not, but my rule is as follows: If any particular rule comes from God, there can be no question that he told us to do it for our own good.