When Mormon originally told of this meeting in Alma 17:1–4, he mentioned Alma’s response, but did not give us Ammon’s response. Ammon was as joyful as Alma, but to the point where that joy “exhaust[ed] his strength; and he fell again to the earth.” This is the powerful presence of the Spirit that Alma had felt in his experience with the angel, and which King Lamoni, the queen, and later, Lamoni’s father, had felt.
In this occasion, Ammon is overcome. Mormon mentions that his brothers were not overcome in the same way, but gives no explanation for the difference. Modern readers may simply infer that different people feel the Spirit in different ways, and that there is no single way to demonstrate that what one feels is truly the Spirit.