It’s okay for your characters not to know the motives behind space aliens. After all, they’re aliens. Space aliens are the ultimate boogeyman for the modern age because they probably do exist somewhere. I’m not saying they actually visit us, but they are possible, as in, not pure fantasy.
"Thank God space aliens haven't figured out distributed systems!"
So, Hollywood, if space aliens invade, and there’s no way for us to talk to them, and we have no idea why they’re here, then that’s all we have to know. You can leave it at that. “We have no freaking clue.”
In reality if something like “Battle: Los Angeles” actually happened, we wouldn’t know and wouldn’t really care. All we’d have to know is that we need to kill them before they can kill us. In JJ Abram’s “Cloverfield” (which in itself was not a perfect movie) no motives were explained nor did anyone know where the alien monster came from. And that’s okay, because there would have been no way to know. Smart as we humans are, there’s infinitely more things in this Universe that we don’t know, than what we know.
Again, turning to “Battle: Los Angeles,” the makers of this movie felt it imperative that they offer some explanation as to why the aliens are invading Earth. But they had no freaking clue, because they didn’t really care — they just wanted to show Marines fighting space aliens. So they made something up and, like usual for Hollywood, it made no sense at all.
The aliens were here for our water. Why? Because they use it for fuel. What?! Really? Well, okay, I’ll buy that. But why fight a heavily armed indigenous species who have nuclear weapons, deep in a gravity well, for something that is freely available all across the cosmos? Their explanation: Because no where else in the known universe is water in liquid form.
What?! Excuse me? For one, that claim is preposterous, but even if you grant that … do these aliens not know how to heat things? You have unlimited water ice in space, which would take infinitely less energy to heat into liquid form that it would take lifting liquid water out of a huge gravity well such as Earth. What sort of stupid backward alien race are these? If they’re that dumb, how did they achieve spaceflight, let alone interstellar flight? And, while I’m ranting, I might as well throw in that any alien civilization that doesn’t understand you should build in redundant distributed control systems in your high tech craft, and onboard AI in your drones, they would not be attacking Earth for any reason whatsoever — they’d be less sophisticated than us Humans, who already have such things, and yet we don’t have interstellar spaceflight.
So, bottom line, Hollywood — just leave it at, “I don’t know!” The aliens are attacking, and your characters say, “Why?!” and they finally conclude, “There’s no way to ever know. Let’s just defend ourselves.”
The one really smart thing they did in “Battle: Los Angeles” is they showed the characters risking their lives to learn enough about the aliens to figure out how to kill them. That is all the grunts wanted to know, and at that point it was all they needed to know. And really, Hollywood, unless you have some brilliant, realistic, scientist-approved idea for alien motives, you should leave it at that.
So, why would aliens want to invade Earth? H.G. Wells laid it all out very realistically over a hundred years ago: prime real estate.
Nothing else I’ve ever heard of makes sense.