Probably the first thing a sentient species has to decide once they venture into space and encounter other sentient species is how to handle expansion.
You might ask yourself, "Why do we have to assume there will be expansion?"
But how could there not be expansion? Let's go through the scenarios.
Suppose the star systems in your vicinity all seem to be uninhabited, but you nevertheless learn that there are other species out there, roaming the stars. You probably don't want them settling down in your backyard, so you need to claim as many systems and worlds as possible to keep them out. If you don't, someone else will!
Suppose you're right up against a neighboring civilization, but there are at least other directions that appear unoccupied. You feel compelled to expand into those, because your neighbor will definitely do it if you don't, as they have the same incentives as you do.
What if you're surrounded on all sides? Now it gets complicated. You might have to size up all your neighbors and figure out which one is the weakest--weak enough to pick off and subjugate--maybe exterminate, depending on your particular ethos. I'm not here to judge. But this could start a regional conflict where all your neighbors try to do the same thing. They may even decide you're the weak link they can go after. This is why you should have expanded earlier.
If you're fully surrounded by a vastly superior race--by which I mean, one with much more powerful technology of a sort that could destroy or conquer you--now you're either hoping they're fully benevolent or you're throwing yourself on the mercy of space tyrants, angling for some kind of arrangement that gets them to leave you alone. Maybe you promise them a steady supply of resources, which they would typically rather get for no effort instead of spending the time and resources taking over your planet. Of course, you're under their thumb, now. They might demand more and more from you, or decide you have something too valuable to let you control. Your home star system could be a promising trade hub, and it's going to be their trade hub, not yours.
Lots of possibilities flow from here. Now you're backchanneling anyone who will listen, seeing if they'll support you if you rebel, perhaps because they have an ax to grind with your oppressors. Those aren't the kinds of allies you can count on long-term, and let's face it, even if you did win there's a good chance you'd get brought to heel by the very people who helped you, for much the same reasons your previous overlords did.
Let's step back to the benevolent overlord idea: it's a contradiction in terms, right? The accumulation of territory, wealth, and power is difficult to justify if you harbor a basic respect for all intelligent beings. You're bound to run into some in your explorations of the frontier, and you have to be ready for them to be hostile, indifferent, or friendly. The worst case being hostile, you need to bring serious firepower. What benevolent species is developing weapons of mass destruction? Oh, sure, they're "defensive." Until they aren't.
Granted, I'm assuming psychology similar to humans here. The fact is most alien species exhibit behaviors humans would consider irrational, incomprehensible, or psychotic. Again, the goal shouldn't be to judge, but to assess threats and establish common ground. A species that worships comets and commits genocide against any "unworthy" races who might lay eyes upon it would at least be justified to stop them from doing that, in a pure "live and let live" sense. Anything outside of that, though, it doesn't matter if they seem crazy. You just need to find a way to get along, or at least convince them attacking you is a bad idea. The funny thing there is most species get very rational once you start adding up things like torpedo tubes, mass drivers, bomber wings. They may not all have direct equivalents from one species to the next, but we're all made of more or less easily-pulverized proteins. A giant rock slammed into your city is going to kill most everyone, no matter your religion, metabolism, or psychological disposition.
It's this basic understanding that underpins all galactic relations. Everyone needs space to keep out of each other's business. Some will want more than others; some will make do with less. Commercial lanes route around restrictive regimes. Weak regimes and contested regions will see lots of piracy. Everyone researches, builds, and stockpiles weapons. Everyone. Even the most pacifistic species get with the program real quick. It's that, or you lose the right to command your own destiny--or worse: you get enslaved or annihilated.
Notably, expansion into space near Earth began with an assumption that no nation could "own" space. It was free territory that anyone could inhabit. This was easy to say since hardly anyone could afford to go there. Symbolic as it was, the United States caused a minor kerfuffle when it planted a flag on the lunar surface, since that's a pretty overt gesture for declaring a territorial claim. In the end, extraterrestrial space (and worlds) came to be owned by whoever got there first and held onto it. What I described early on happened in microcosm, just among humans. Of course, humans had already done this all over Earth, and it brings us around to the likelihood that only civilizations who aggressively claimed territory and exploited resources ended up making it to space where they could do the same thing. After all, if you're happy in your little corner of the world, what reason would you have to go tear up another? But if you're constantly craving more, the only option is to go out and find it.
It's not all bad, of course. The Oolians are about as close to benevolent as one could get, and most of their problems are the problems of a massive, aging bureaucracy: calcified decision-making, resistance to change, institutional inertia and self-justification. When the chips are down and somebody comes stomping around the neighborhood, they're still usually there, sweeping them away like so many ants. They can't get involved in every brush fire, and in fact often display an insultingly paternalistic attitude about not wanting to "pick winners and losers" in the galactic struggle for supremacy. This is well and good to say, but it's obvious they'll do whatever it takes to ensure they are the winners, when all is said and done. Everyone that isn't directly an enemy of theirs ends up acting as a buffer against those who do have bones to pick with the Oolians. Still, they do show up and defend their buffer-protectorates. Usually. For now.
Earth is, unfortunately, caught in that "surrounded on all sides" scenario. We hold a couple dozen solar systems, though only a handful are actually colonized in any meaningful way. Most of what we have was either weakly contested by other empires, unwanted by anyone (that is to say, lawless), or was held by the Cleronids until their sun blew up and swallowed them. Our experiences with the Koraxians showed we don't have any real ability to hold this territory in a stand-up fight, not unless our enemies are total chumps. (The Cranions, for their part, weren't chumps, and gave us a real run for our money, just not the absolute ass-beating we got from the slimy ones.) So, what to do? Weapons research only moves so fast, the Oolians only share so much, and diplomacy only gets you so far--even nowhere when it comes to certain boneheaded governments.
And this is why the ability to travel through time and just headshot the Vorchons' equivalent of George Washington or whatever is really just the best.