Why are wartime comrades so close?

It is a story often repeated. People serving together in a military unit engaged in combat develop strong ties to one another and maintain contact after leaving the military. I have heard it said that the perspective when fighting is not that of serving some great cause or defeating some evil enemy, but rather of protecting the members of one’s unit.

Has anybody ever studied this phenomenon? Does it occur anywhere else other than in combat? For what it is worth, let me float the following idea. Engaging in combat must in some sense be part of our nature, but so is a repugnance toward killing. I wonder if part of the closeness is a kind of shared guilt.

There are other situations I can think of that have some of the characteristics of fighting together, but as far as I know none of them has the same tendency to create lasting friendships: People on a sport team, either amateur or professional or people engaged in dealing with the consequences of some natural disaster, policemen working together.