My space opera stuff is all still in its building and outlining phase, but every once in a while I do a big push of figuring stuff out. And when I do, I always get a sense that the scope STILL isn't big enough. For example, I've roughly defined the area within a 100ly radius of Earth (roughly 4.2 million cubic light years), which includes 4660 stars. Off those, 1568 stars have planets, 361 of those have life of some sort, and 153 of those have intelligent life. And of those 153, 71 have achieved interstellar travel by the year 2373.
(Excel spreadsheets and some extreme dorkiness on my part are responsible for all this information.)
One thing I asked myself is how one can apply the lessons from Guns, Germs and Steel on an interstellar scale. It's a challenging thing to speculate, as how can you tell what resources will really make a difference on an interstellar scale? Do germs really matter at all?
But one thing that became clear as I mapped stuff out was this: who your interstellar neighbors are matters. Because the technology difference between "capable of interstellar travel" and "not capable of interstellar travel" are so extreme, it would make Pizarro's defeat of the Incans seem like a balanced fight. Once interstellar travelers come upon a planetbound species, what they decide to do defines the entire encounter. If they're genocidal conquerors, then the planetbound species will be eliminated. If their imperialists, then the planetbound species are now part of the empire, full stop.
So I decided, for things to make sense to me, Earth's neighbors had to be preservationists. They had to be of the mindset that when you encounter a lower-tech society, you might do a little clandestine research for the sake of science, but you otherwise leave them the hell alone. Perhaps even a step further: they had to have just enough militant in them to draw a line and defend a defenseless species from an invading force.
Using that knowledge helped me define our immediate neighbors, as well as humanity's role on the interstellar scene (which is more or less like a teenager who is smarter than he's wise on his first internship).
Of course, the same logic applies when two interstellar species clash. If you have one species with no respect for alien life who will commit acts of genocide without a moment's hesitation, then the species they come in contact with must devote themselves to defense. They have no other choice.
But it's more than just wars and genocide, of course. You want alien species to work together and cooperate, or at least trade. Because if you don't, how else can you get a cool cantina scene?