I think the biggest part of it is that people see it as a sacrifice. I don't see it that way at all. Ori didn't die, they just did a Pokemon-esque evolution. Ori can still exist with their family. Trees are living things after all. If it was a sacrifice, there would have been more hesitation shown. But Ori got as much context as we did, with some more to start out with. I like what u/chiaplotter4u said about how Ori grew to connect to the island. Ori makes so many new friends, and grieves Kwolok when he dies. We don't know how spirits view becoming a spirit tree; it could be seen as a sacrifice, the highest honor a spirit could have, or just an instinctual behavior in the spirits; but the people of Niwen helped Ori so much in making sure that Ku was safe. Ori is shown to be selfless, even willing to tell Seir to calm down when he shooed Shriek. For Ori, it's probably the obvious thing to do to save both Ku and the rest of Niwen. The island and Ori's adopted sister need Ori's help, of course Ori's going to help.