Yunalesca's speech explains quite clearly why the people of Spira haven't become cold, detached psychopaths. They always had hope. For a thousand years Sin has been killing them all and for a thousand years they were promised it wold end someday. They believed this promise so to them, Sin wasn't "an inevitable part of Spira's destiny." It was something that could be fought and conquered. They truly believed this.

...only it couldn't. At leas tnot the way they were told. So with every destruction of Sin they gained about ten years of hope that this time it would be gone. Only of course it wasn't.

Most people don't want to die and even more people don't want their families to die. Living under the constant threat that you and everything you lov ewill be destroyed is utterly depressing. And the people of Spira had to endure that for far longer than any other world in an FF game.

I believe my relatives went to Heaven. It doesn't mean I'm okay with them dying. It doesn't matter what afterlife awaits you. Seeing someone you love die is always going to be heartbreaking.