Hussar
Legend
Oh, sure. I agree it wouldn't be automatic. Of course not. And not 100% true either. Sometimes it would work, and sometimes it wouldn't.I get where you're coming from. But looking at the past few thousand years of human history, we're an intelligent species that does not always get along with one another. I wouldn't take it as a given that a treant would automatically get along with the humans who just moved down the creek a few years ago.
The problem is, as I see it, D&D worlds never take the times that it does work into account. And, again, we're not talking about a couple of years either. These would be communities that have lived side by side for (well human anyway) generations. It's not like pre-industrial people were all that mobile, by and large. If you were born in the south of England (say), you probably lived and died within a single day's walk of your birthplace.
It's not really unreasonable that various communities, (remembering that human communities are only one of many) would begin working together for mutual benefit. It's pretty rare that neighbouring communities are constantly fighting (certainly not unheard of, but, generally the minority) for generations.
Again, that's neverminding things like elves.