Mormon says that Aaron was rebuking his brother. Ammon had said nothing offensive to Aaron until verse 9 when he seemed to take credit for their success, saying, if we had not come up out of the land of Zarahemla. Aaron's rebuke is actually quite gentle, I fear that thy joy doth carry thee away unto boasting. Aaron is saying, "now don't get carried away thinking you are anything special." Ammon was, of course, "something special" but a true servant of the Lord never takes credit for what the Lord has done. To do this demonstates pride and insolence. Aaron wants to make sure that Ammon does not give himself too much of the credit. But Ammon never meant to extol his own virtues and his response is a great explanation of his own nothingness compared to the power of God.