My mind is on edge at the moment. It seems to be flitting from thought to thought with little consistency. One moment I'll be thinking about Battlestar Galactica, the next hamsters. It's just been the way things have gone. There is an advantage in this: I've been getting quite a few creative ideas.
The way my mind's flitted from place to place lately, browsing TV Tropes isn't the brightest idea. Within seconds of clicking on a trope about dialogue I could be perusing a very different trope on the nature of the apocalypse. However, with being an SF writer I tend to keep my trope browsing to the SF realm. And today I came across a beauty, which got me thinking.
My thought was this: Why would an alien race want to invade the Earth?
The Martians in The War of the Worlds had a good reason to pick the Earth as their target. They were living on a dying planet and needed to move. Earth happened to be the nearest convenient Goldilocks zone planet, so they invaded.
Less clear-cut was the Slitheen in the first series of the new Doctor Who. Their reason for invading was to reduce the Earth to molten slag so they could sell it off to the highest bidder as fuel. Only, they could have done this to any number of uninhabited small rocky planets anywhere in the galaxy. Mercury, Venus and Mars are all candidates in our own solar system. We also have dwarf planets such as Pluto (though the destruction of it following its demotion would be overkill, and possibly set the entire astrology community to a path leading to perpetual civil war), Ceres, Makemake and Eris which would fulfil criteria. None of the aforementioned planets have life, you'll notice, and neither do any of the dozens of moons in the Solar System. Why didn't the Slitheen just go to one of those? Yes, we might end up narked that some alien race nicked a planet, but (other than altering the entire gravitational balance of the Solar System) what real harm would that do?
Resources provide no excuse for an alien invasion. Water is available in colossal quantities throughout the universe (it just needs thawing out), as are uranium, iron, hydrogen, oxygen and any number of vital materials. Even lebensraum isn't a great exuse, as if an alien race is capable of building an interstellar fleet then, by and large, it should be capable of constructing habitats on uninhabited planets such as Mars and Venus.
It's fair to say I've never been a great alien invasion fan, especially when I'm expected to take them seriously. Only occasionally is an invasion really justified for the purposes of the plot (a mental religious attitude might justify it) and even then I doubt it'd be something I enjoy.