John Wesley on Deuteronomy 27:15

Cursed - The curses are expressed, but not the blessings. For as many as were under the law, were under the curse. But it was an honour reserved for Christ to bless us; to do that which the law could not do. So in his sermon on the mount, the true mount Gerizzim, we have blessings only. The man - Under this particular he understands all the gross violations of the first table, as under the following branches he comprehends all other sins against the second table. Amen - 'Tis easy to understand the meaning of Amen to the blessings. But how could they say it to the curses? It was both a profession of their faith in the truth of it, and an acknowledgment of the equity of these curses. So that when they said Amen, they did in effect Say, not only, it is certain it shall be so, but it is just it should be so.