Sforno on Deuteronomy 26:18/ספורנו על דברים כ״ו:י״ח

1וה' האמירך היום, by entering into the covenant with you, a distinction He has not accorded to any other nation.

2להיות לו לעם סגולה, so that He may achieve with you what He hoped to achieve with mankind when He created man, saying: “let us make man in our image, etc. (Genesis 1,26).

3ולשמור כל מצותיו, He accorded you an additional distinction in having chosen you to observe all of His commandments; by comparison the other nations have not been rated important enough in His eyes to warrant these commandments. Proof is the statement of our sages that if a gentile decides to observe the Sabbath legislation voluntarily as a religious act, he is guilty of death, as ever since the first sin, man had been condemned to labour in the sweat of his brow for a livelihood, and only the Jewish people have been relieved of this decree on one day of the week, i.e. the Sabbath. (compare Sanhedrin 58) All other nations have been given only seven commandments to observe.