… Why should we care if someone adds more Mitzvos in order to give Hashem more than he asked for? And if it is not good, then the order in the Pasuk should have been: Not to subtract and then, not to add, as subtracting is worse than adding.
Chazal explain that in order to avoid subtracting from the Mitzvos, first comes the warning of “not to add to the Mitzvos.” Because when you receive Mitzvos from heaven, there is nothing missing and there is nothing extra. If you add anything, even with good intentions, especially if seemingly it was successful, now you can come to subtract from the Mitzvos as you say to yourself, I was successful in adding, now I can trust my smarts and take something off as I see fit. Therefore, the Torah warns you first about “Not adding…” [because that is how it starts.]
— Igros Kodesh vol. 32, p. 163.