D.Ethos, or an appeal to ethics
Ethos appeals to one's character.
Many topics from "The Things They Carried" are still relevant today, the constant theme is the horrors of war, and how they affect ill-prepared men, both physically and mentally. Many of the items they carry are symbolic as well as practical, used as a reflection of their own battles, and serve as a bridge between their fantasy world and reality.
Ultimately, their psychological baggage becomes the hindrance that leads them to their death.
He says this to reassure ralph that things will be ok. Simon is one of the most sensitive characters in the book and becomes aware of Ralph's worry. This is also an example of the books theme of herd mentality and social reliance since simon is aware of the effect his words and examples have on the others.