Yep. Except (IMO) 2 and 3 are just proxies for 1.
It could be because of a race’s natural characteristics. In Niven & Pournelle’s The Mote in God’s Eye, the Moties, having reached sexual maturity, have to breed or die. This means their history is an endless cycle – “Cycle” – of overpopulation crisis, civilizational collapse, and recovery. This, however, has been confined to their home system; they have never achieved interstellar travel because in this universe, FTL jumping is possible only at a natural “Alderson Point”; there’s one near every star, but in the Mote system’s case the nearest one is inside a star. Once they make contact with the Empire of Man (which has the forcefield technology to make jumping into the Mote system survivable), they are at great pains to keep their reproductive problems a secret. They fear with good reason that if the humans find out, they will exterminate the Moties, for fear that, if the Moties ever do break out of their home system, they will fill up all the habitable worlds in the Galaxy – exterminating humans in the process – and repeat the Cycle on a galactic scale.
That is the assumption of Heinlein’s Starship Troopers: No matter what casus belli is stated, all war is essentially a struggle for Lebensraum, and this holds true as between sentient species and on a cosmic scale. It makes no difference whether humans and Bugs can learn to like each other; what matters is that they both want – essentially, need – the same planets to live on, and can’t share. Any race that doesn’t accept this, and tries to be pacifistic, is doomed to be wiped out eventually by a race with fewer scruples. It’s a rather fascistic worldview, when you think about it. (Otherwise, however, the human Federation, in its internal organization, is militarist but not fascist; the government is actually rather minimalist and libertarian.)
Or it could be cultural. In S.M. Stirling’s Drakon, the (human-drived, genetically-engineered) Draka conquer and enslave all alien sentients they encounter, and plan ultimately to conquer the whole Galaxy, because, well, because they’re Draka and that’s what they do. It’s fun! 
Essentially the same thinking motivates the whole human race in Birthright: The Book of Man, by Michael Resnick.
The laws of physics do that. This is GD not Cafe Society; so my assumption is that such things as FTL drives aren’t a factor unless the OP brings them up for a “what if” question.
Heck, I’d say this discussion pretty much presumes we’re in sci-fi land; the mere act of coordinating an insterstallar war presumes that you can get materiel and communication back and forth in reasonably good time. How much of a war is it if it takes decades or centuries to give and confirm an order to attack?
The only plausible war scenario I can think of that isn’t excluded by the physics against FTL is the ‘roving colony ship’ scenario: you fly to a system, upon arriving discover that it’s occupied, and process to attack the locals Independence Day- or Battle For Terra-style. But the thing is, that scenario seems to go against the question of the OP, which asks why you’d attack if you could just go elsewhere in space for what you want - if you’re the colony ship, you’re attacking because you’re there and finding other territory/resources would be obviously costly or inconvenient.
Okay for the purposes of the scenario assume that some sort of FTL type (stargates, wormholes, hyperspace, magic space ponies etc.) technology is attainable in the future.
This is a joke, right? Realistic? Interstellar/interspecies war?
Ummm, ch’yeah. OK.
Well, several of the stories in the shared-universe Man-Kzin Wars books deal with how you handle a sub-lightspeed offensive. I’m sorry I can’t get into more detail, as it has been quite a while since I read any of them and I don’t know if the books made the latest move with me. I think the highlights included having to send far more material than you might think is needed, be slow to adopt new changes due to the issues of maintenance and supply, and give great autonomy to the commanders. I believe the most detailed is Donald Kingsbury’s “The Survivor”, but I’m happy to be corrected.
I like the Posleen series by John Ringo (Legacy of the Aldenata), which speculates that there is an alien species that are basically interstellar conquistadors. Their motives are both to destroy (and eat) all other intelligent species to protect themselves from attack, and to colonize, exploit and then move on from any system they take to continue the cycle. Since they breed at such a high rate what it boils down too is a couple of generations of peaceful exploitation (after they have wiped out and eaten all other sentient species in any given system), with the resources of a conquered system being used both to rebuild an infrastructure of their own and for the richer ones to refit and move out to other systems, followed by escalating war among the conquering clans that remain as they use up the resources, followed by basically apocalypse as they destroy themselves and the habitable planets in a system and the survivors move on.
Sort of like interstellar locusts. They don’t really attempt to coordinate an interstellar war so much as build up, identify new targets, then independently (or by clan) move out as the spirit takes each individual to fight, conquer and do it all again. It could be a good strategy for a race, given the right mind set and motivation, even if no FTL tech exists, as long as they have the technology for hibernation or generational type ships, assuming that habitable planets are relatively scarce and thus are valuable.
-XT
Suppose our planetary coalition of governments is badly split on the allocation of resources within our newly settled off planet colonies, and the economics of empire begin to catch up with us. We start to find out that the colonies most able to pay back the development costs of their newly self supporting populations are the same ones more inclined to embrace the concept of liberty, and self determination, not to mention self investment.
Let us further suppose that an alien culture is detected, perhaps even contacted, and our own interstellar explorations are a bit hemmed in, although not particularly hampered by the geometry of our contacts. While the aliens are not specifically hostile, they do want unobtanium, and are eventually going to compete with us for it. Ok, maybe not compete in a military sense, but compete.
The facts of our own history show that even an economically unfeasible, and disastrously expensive conflict over technologically large distances can be started without any particular hostility on the part of the actual combatant enemies. Homeland politics drive armies. And the fact that the systems being fought over have no known reserves of unobtanium will not matter at all. If opposing domestic political parties see and advantage in interstellar conflict, one side can start a war. Might be them, might be us.
Tris
A likely reason? Ideological conflict, like the Culture/Idiran War of Consider Phlebas. It’s the only reason for interspecies conflict for starfaring species - such species IMO being necessarily post-scarcity unless, like the Idirans, they have a necessarily-planet-living religous thing.
Subsets include religious jihad & hegemonizing machine swarms, but the central theme is still ideological differences.
I think people need to remember that alien cultures are, well, alien.
Who knows what will motivate them? What will concern them? What will drive them to be homicidal maniacs?
It is fundamentally unknowable. Hence the speculation.
The only “real” reason I see is a fight over resources. If habitable planets are rare or there is an unobtanium planet or somesuch.
Beyond that it can be anything from a misunderstanding (didn’t realize that hand looking thing was your head when I shook it in greeting) to a literal genetic or cultural imperative to wipe out anything not them.
It doesn’t have to be about resources - it could just be We know they have the capability to hurt us, and can’t be sure they won’t, so let’s get them first - on both sides. Mutual distrust.
No interstellar-faring species with a brain should be fighting over planets. Planets are for primitive dirtgrubbers.
Boredom?
Too many in the US viewed Gulf War 1 as a kind of spectator sport.
I could picture an old civilization seeing a yopung one as a plaything/victim.
A schoolyard bully, that commits genocide.
On film. For a profit.
Concur - if you can travel between the stars, you’ve already solved every problem for which a planet might offer a solution.
And if we travel to a planet where the main sentient race is a silicone based thingy that looks like a very slow moving rock that needs a particular world … we roll i n and do not recognize them as sentients and terraform the world to our needs and kill them off.
Who is to say that there would not be a race that does not consider a sentient race any reason not to take a world and chtorriform it to their specs
Yep, if they are tasty then we will want to eat them. And vice versa. That’s what “interspecies war” is mainly about now, why would interstellar interspecies war be any different?
One possible reason to launch an attack on an alien civilization would be to take a technology or a technologically advanced machine.
As an example, if one race had developed antimatter rockets but the other had not - it would potentially be worth the risk to seize such spaceships and use them, as well as trying to duplicate them. Such events might precipitate a war but at that point both sides are on more equal footing.
I imagine wars would consist of simply sending huge nuclear strikes on the other races planets and offworld bases. Why bother with conquest - just destroy the other race until their threat is eliminated. As mentioned above - nothing material they have is worth the expense of the space travel.