People would naturally be more concerned about themselves than others for survival purposes, as Darwin might have reasoned. Quite a lot of human psychology stems from the Survival-of-the-Fittest notion.
"I'm gonna get a tattoo to prove that I was in the Navy." "Oh? What are you going to get tattooed? An anchor or a life preserver? Oh, what about the battleship SS Missouri?" "No, my mother has that." "Your mother has a battleship tattooed on her???" "Yeah, whenever she wears a backless evening gown, it looks like it's sinking. I know what I'm going to get as a tattoo." "What?" My uniform."