I have a friend who was a submariner, and he explained it this way to me:

imagine being stuck in a super cramped boat, having to share the same tiny, cramped bunk bed with other crew mates (they sleep in it on their sleep time and so a bed isn't empty and wasting space), and you do it for months at a time.

and you do this all with the same group of guys, so you better hope that yall at least pretend to get along, because it's a long ass time trapped in there.

(I think he had mentioned its one of those "no girls allowed yet" jobs still because their mission parameters basically put them needing to be submerged for literally months and no access to offshore medical)