Why does trying to keep the Law to earn God's favor put you under a curse?

Les explains that when a person says they will keep the Law in order to merit favor with God, they are warned there is no way any mortal man can do it, so they put themselves under the curse. He points to Deuteronomy 27: “Cursed be he that confirmeth not all the words of this law to do them.” That means if you do not keep every last jot and tittle of the Law—as no man can do, and only God can—you are under the curse of God. Les then connects this to Galatians 3, where Paul says, “For as many as are on the works of the Law are under the curse, for it is written, Cursed is every one that continueth not in all things which are written in the book of the Law to do them.” The warning is: don’t put yourself under the Law, since as a human you cannot keep it. Putting yourself under a so-called legal system actually puts you under the curse of God. Les says this is serious business and serious language, and it is not his but the Book.