This is an edge case. Marines in particular, and the military members in general, will form a special bond that can seldom be duplicated elsewhere. And that has been my experience. I was among a group of six that met at the schoolhouse after boot camp. We did not get sent to the same units, but we were all PCS'd to the same base. Two of us (not myself) made a career of the corps, and the others went to school after s…
Externally, it seems this bond could be explained by: 1. Everyone is together, away from distractions, and unable to leave easily for months on end 2. You are working toward shared goals 3. You are friendly toward each other It's a recipe for a strong bond. Another example of this is famous bands. They're constantly traveling for work, together, trying to write more music and become more famous as they go.
Many civilians are aghast when they see how Marines act toward one another. The constant shit-talking and physical fighting probably wouldn't be described as "friendly" but definitely "brotherly"