has some level of sentience and is actively hostile to invaders? ("Here There Be Tygers" by Ray Bradbury is one example.)
I was just thinking of something similar -- a planet that's quite beautiful and capable of supporting human life, but which resists human colonization through more subtle means. Practically no one stays on this planet for more than a week, because something about the planet activates the stickiest, most annoying earworm in your brain and plays it over and over and over again until you want to run shrieking. The only people who can successfully colonize this world are the tiny contingent of spacefaring humans who were born deaf.
So you might well say that, under the circumstances, it's a small, small world.
Yeah, I'm evil.