Like Brian said, fun means different things to different people.
I always say we can't bash paintballers and speedballers for doing their thing, because they and real steel shooters alike bash us for doing what we do lol
Like Lindz, I'm in this for the challenge and competition. And as he said, it's not that we purposely portray soldiers, or want to be pretend war fighters, but the best way to organize ourselves and train in our sport just happens to be extremely similar to real military stuff.
Our hardware emulates theirs, and so the tactics and training are naturally also very similar. The whole basis of airsoft really is to emulate real world counterparts, otherwise we'd all have fantasy guns. The line is really determined with the individual, whether you couldn't make it in the military so you do this, or you want something a bit more serious than paintball, or you see it as a sport.