You also raise the interesting idea that military organizations basically trick soldiers into treating the members of the unit like biological brothers. That may be a plausible explanation for why the Marine dove on the grenade. But it cannot explain why a firefighter would enter a burning building to save someone he has never met.
Well, maybe an old ex-smoke eater (breathing devices were just starting to appear) can help.
The story begins as a little child, the first of a whole herd of kids, born changing baby diapers if I can still remember that far back.
I was taught that my job was to take care of the little ones, protect them and that also meant I had to protect and take care of their little friends as well.
During my mid to late teen years I became a Volunteer Fireman (the only kind we had at the time) and the training continued. The idea of priorities was stressed, that the priority was save lives first, property next. Lives were always paramount.
Many years of training followed eventually becoming one of the Instructors for the University of Maryland's Fire Service Extension System when I then trained others.
Is there a Genetic component to such behavior? Possibly. Of the passel of kids one brother became an ambulance first responder (we didn't have EMTs way back then) but none of the other brothers or sisters.
The biggest influence seems to be training, training from a very early age that I was responsible for the safety of others, and then lots of training on how I could do that with the least risk to myself while still doing my duty.
Yup, the "D" word.
Anyone so limited that they can only spell a word one way is severely handicapped!