The astronauts of the early programs were basically all picked from the military test pilot pool. Which was a natural fit given how everything related to spaceflight was new and experimental and ridiculously risky. They needed people who could stare death in the eye and live to tell the story.

These days astronauts routinely spend months in space, and indeed their selection process and education requirements are rather different—as of course is the whole purpose and character of the manned spaceflight program.

It should be said, though, that Neil was selected as the CDR of the first mission to land exactly because he was not a gung-ho type but instead extremely calm, collected, and not one to make a number of himself. Buzz, on the other hand, was and is a much more hot-blooded personality.