Obviously, The Revenant has garnered critical acclaim, so everyone who doesn't get it (which may actually be most audiences) is going to be up in arms at that fact. If you're familiar with Iñárritu's other work - in particular Birdman - you'll recognize that his style lies in the Latin American tradition of magical realism. So the plot, which detractors have called slim and unoriginal, is not actually the story. There's something else going on, if only you can unlock the hidden meaning. I think I have, and its keys are 1) in the title, and 2) in a strange speech given by Tom Hardy's character. Always look at an artistic film's strangeness closely, that's often where the decoder ring is (or really, in asking "why").
Quick synopsis before we apply that decoder ring to it: 1820s frontiersman Hugh Glass (DiCaprio) guides a fur trapping expedition gone wrong with his half-Native son when he is struck down by a bear, which he kills, but not before he his grievously wounded. The expedition's captain (Domhnall Gleeson) knows he'll die but wants him to have a proper burial, so he leaves three men, including Glass' son Hawk and the antagonistic Fitzgerald (Hardy) who ends up killing Hawk and leaving Glass for dead to save his own hide. Glass survives, however, and undertakes a quest to find Fitzgerald and take revenge, encountering several more lost souls in the harsh frozen wilderness and surviving through it all. When he finds his son's murderer, he actually doesn't take revenge, but leaves him to a Native tribe who do the job for him. Only then does he allow himself to die.
There's a definite play on totem animals. Glass is almost killed by a bear - a mamma bear protecting her cubs, just as he is devoted to protecting his son - but he kills it. From then on, he wears her skin and takes on even more of her qualities, including a ferocious indestructibility. He runs TOWARDS danger, just as she did. Compare to Fitzgerald who, in that strange speech, talks of killing "God" in a squirrel (his father does, and replaces God, making him a squirrelly Christ figure in his own mind). For him, it's a statement of man's power over nature and prefigures the death of the land and its people. In this animistic fable, it's literal truth, since the Spirit of the land is in each animal, and a god. In relation to Glass' bear totem, Fitzgerald is a squirrel, running and hoarding his way through the narrative, for that is the "spirit" he killed.
Glass is able to navigate the Land and its dangers only because he is Pawnee as much as American. He turned his back on his origins, married a Pawnee woman, had a son with her, and killed an army officer to protect them. But his village was wiped out and he's been forced to work with the white man again. His (adopted) people are gone, but he is apart from those he was born from. But the corruptive nature of European values which has already killed this reality (certainly from our vantage point up river in History, see also the point of the buffalo herd) does threaten to turn him back into a "Christian white man". It's perhaps the reason why his wounds evoke Christ's scourging (as well as his resurrection), and why the comparison to Mel Gibson's The Passion of the Christ can so easily be made (there's also a scene where his hand is pierced if you want to play with those comparisons a little more).
Came out of the theater with all these ideas spinning in my head, shared them with my friends, who added a couple of elements you've read here. My thanks to them, and to you for sticking with me this far.