Reasons for mitzvot is from the Gemara itself. Bava Metzia page 119 and all rishonim like the sefer haChinuch.

And the reasons are not thought to be mysterious. After all that is the whole point of the argument between R. Shimon ben Yochai against and the sages. he says when the reason does not apply the commandment itself does not apply. If the reason for any command was mysterious then there would be n place for the opinion of R Shimon ben Yochai.

[That is the characteristic of  all arguments in the Mishna. They disagree only about the specific point they are talking about. R Shimon holds we go by the reason for the verse. The sages say we go by the literal meaning. But there is no argument if we know the reasons.

So the commandments are to bring to these basic things: good character traits, peace of the country, to minimize the physical desires, to get rid of idolatry.

In short this could be called natural law of what Rav Saadia Gaon calls חוקי השכל. [Laws of Reason]

So no where in the rishonim do we find Divine command theory. [That is the theory that says: the mizvot are good because they are commanded. Rather they are commanded because they bring one to good.]