This is the most I've been able to come up with....
Everything, everyone will die eventually. Everything is in motion all time.
The body will wear out. I think we can figure out and overcome the aging process at some point but we haven't yet. You get old, things wear out. By the end I'm thinking it's like having an old car, a royal pain. Add in some pain and things not working well or at all (lower quality of life, less dignity) and death will probably be a nice alternative. Nothing being better than suffering. Although when the times comes, I'd probably still pick suffering over dying but eventually.... that's going to get old and I think I'd just want out.
I imagine it's probably not that bad. Probably like going to sleep. Plus, if your brain floods the body with things that feel good, it might not be so unpleasant. Suffering for years before death sounds worse. Or losing your mind or having your quality of life drop hugely.
You go to sleep, you don't remember anything. If your brain/body is no longer functioning, you've got nothing to perceive with. In the "normal" sense you won't be aware of anything.
I think we're "our bodies" too, but we sit mainly in the brain. You can lose a limb and you're still you. You can't lose certain parts of your brain. You won't be there anymore. Somehow the brain retains memories. If the brain shuts down when you die, I wonder if -- if you still exist after death -- if you still retain those memories.
Maybe we exist as energy or something beyond what science can detect. Maybe another dimension. who knows? Maybe we just become energy after death but don't really do much with the physical world anymore. Maybe we're somewhere else. Maybe we're something/someone else, reincarnated. Or maybe there's a soul that captures the essence of our experience/memory while we're alive.
No one else knows. If they did, everyone would know about it.
Animals have personalities. I agree on that. Some of those animals taste good too.

I would imagine the same thing that happens to us happens to animals. I'm not sure where the cut off is though -- Consciousness? Insects? Plants? Fish?
It's going to be tough to prove anything. If someone has a vision, even if everyone has the same vision, it can be explained by the brain dying. Bright light? That's just what the brain does when it runs out of oxygen. You'd need many people all independently saying the same thing, something that they couldn't possibly know otherwise and where each person knows the same thing. Bright lights don't cut it.
And if no one else has figured it out by now -- And there's a great interest in it by everyone -- it's doubtful you'll figure it out either. Not that anyone will stop trying though.
Nobody's ever come back. The most we can do is postpone it a bit. Even then, with healthy eating and exercise, how much do you really gain? Maybe sixty years at the most? Dying at age 60 vs. age 120? Nobody's living past 120. Even if that edge gets pushed... It's still going to happen anyway.
Even if they figured out some way to keep people alive... Then what? The planet has seven going on ten billion people as it is. We'd have to get off the planet probably, unless we existed as software or something that's not taking up space and resources. Technology takes awhile to figure out... Are they really going to figure out and overcome death in the next 100 years? And even if they do, are they going to pick Joe Blow to preserve?
I'd still be in favor of anything that would put it off though. Although I've heard even if we lived forever, after about 400 years statistics catches up and you'd get hit by a car or something. An accident.
Another property of death -- Permanence. That person will never do anything more. Nothing will ever change in their life again. Everything is done and frozen. And the living can't communicate with them again. Forever. Or until their own death since we don't know what happens, but at least in their lifetime, they're not communicating with the person who died again.
Afterlife? Next state? Nothing? Who knows?