For lots of us it is as close as we will ever get or want to get for that matter to real combat. Plus you get the chance to shoot your buddies.
It is as serious as you make it. When I am in game I try to act as one would in a real situation. Or at least how I feel they would act. But at the end of the day it is just a fun game, fantastic stress relief, great excercise and an over all good time.
Play with Honor. Call your hits.
Nothing just happens, You always have to strive...