Celebrim
Legend
Except these aren't alien races.
I disagree. They share a world, may possibly share overlapping ecosystems, and may face the same threats or worship the same gods, but they are aliens living together (or frequently, not living together).
Sure there are distinguishing features (longevity, native habitats, maybe relationship to magic) but there's no reason to assume that what we think of as "recognizable human behaviors" aren't the same as "recognizable behaviors of intelligent life."
How many sentient species other than humanity are you familiar with? So on what grounds do you think that "recognizable human behavior" is any way the same as "recognizable behaviors of intelligent life"? What are the recognizable behaviors of intelligent life that all intelligent life has in common? Is there any way to give a definitive answer on that?
I think there is every reason to assume sentient species have radically different outlooks.
I would expect overlap in as much as there is overlap in the 'biology' of the two species, but where that biology differs there will be very large divergence in outlook and behavior. How much the biology differs is something of a matter of opinion, because to a large extent the ecology of an elf is an additional construction. If the biology doesn't differ in any great degree, which is a fairly common assumption, then you aren't doing it wrong if they don't differ more than 'humans with bumps on their forehead'. If the biology however differs a good deal, then yes, you are doing it wrong. There is nothing wrong with claiming that elves and humans are basically the same and therefore the expectation is that they ought to act the same.
My problem comes from trying to do both - radically different biology but no corespondingly large difference in perspective or behavior.
This comes up quite a bit in my games just approving the concepts for non-human characters players come up with. Elves is particular cause all sorts of problems with failures of the player to really come to grips with 'elves are different and I mean it'. As I've drawn them, they don't have the same emotional contexts (they barely understand sexual lust, for example), they don't have the same metabolic needs (beauty and freedom are physical needs like oxygen and water and depriving an elf of one kills it), they don't have the same understanding of time (childhood lasts a century, most anything they could possess materially is going to decay before they do), they don't have the same relationship to the natural world (you have to be ok with basically being able to talk to everything you eat), they wouldn't consider it abandonment or bad parenting for a parent to kick their '8 year old' out of the house to fend for themselves for a year or two (in fact, that's normal), and so forth. I get well-intentioned players that write backgrounds that are literally impossible for an elf but make perfect sense of the character was human. I've had to write a 6 page handout for, "So you want to play an elf?", and still have to point out that its not possible for an elf to have been a slave.
As a personal preference, I feel that if you are going to have elves be humans with bumps on their forehead, you add nothing to your setting and would get a richer setting if humans were the only race. That is, the central reason I can think of for having a nonhuman race is if it provides a nonhuman perspective. I suppose that there can be some sort of trivial mechanical reason, 'this race has +2 dex', but all that could be just as easily folded into more diverse human character creation options.