Concept.
Alien species reincarnates after death, retaining most memories. In this process, their biology and psychology adapts, resulting in some random minor alterations as well as a stronger resistance to whatever killed them the last time (homesickness, burns, a specific disease, etc.). They are fragile.
Due to this, they don't process death anywhere near the same as humans do. Depending on how reincarnation works, either you'd have to near-completely destroy the corpse (i.e. they arise like phoenixes - maybe they are phoenixes) to stop them, or they straight-up always come back (i.e. their souls go to their god to be reborn). A wild animal might not fully eat the corpse, enemies in war would only want to incapacitate their foes since that's more humane and the foe might join their side on resurrection.
So, there's not much chance of someone dying permanently - and as a result they don't have much of an ingrained response to this, no immediate emotional reaction, just a dawning emptiness over the course of months or years.
When a human friend dies of disease, they look at the corpse, and they're a bit confused on how to feel. They know this is normal for humans, but it's strange. They know the human will never come back, but their instincts don't know that. And it takes them a very long time to fully, internally understand that the human is gone.
