A lot of ways to die are excruciatingly painful, but you die, so you don’t live with the pain. If you end up in one of those situations and don’t die (because you are immortal), I imagine the psychological impact of the pain without immediate release could be enough to completely break you, mentally.

At some point, our sun will go supernova and you will end up drifting through space.
And all your life before that point will be less than a blink of an eye compared to the time that follows:
Trillions and trillions of years until the heat death of the universe.
And even that time will be less than the blink of an eye compared to the eternity afterwards, when you drift through a black void without any stars.

If you have epilepsy or Parkinson’s or MS, you’re just going to likely get worse forever.

All the comments assume everybody else isn’t also immortal. I forget the title and author but there’s an old sci fi story (or novel?) about a future where everybody lives for centuries, and they’ve found that the brain only retains a certain amount of experience. They have long careers, get tired of doing whatever, re-educate and do something else, or even have multiple families they eventually forget about. A couple of the characters are surprised to find out they used to be married like a century earlier. To me that seems vaguely like reincarnation, and I kind of don’t hate the idea. I really don’t see any downside to that scenario, or even just going on forever.

People are focused on having regrets and negatives that last forever. But buck up li’l camper, you can learn to move on from stuff. And I say this as a dad whose daughter had cancer at age 10 (she survived). It was hell and I wouldn’t want to live through that whole period again, but I don’t consider it a reason not to want to live forever. The trick is to learn how to cope with these things and not let them outweigh the good experiences you have.

Idk id be super depressed if I was able to experience my family, friends, family’s children, and so on die.

LoL investing and compounding! I literally have all the time in the world.

If you’re injured and you survive with the scarring from said injuries. Well, good luck because you’re now going to wear those and wish you had died from them. If you’re incapacitated or amputated? Gotta live with that for years and years.

Life will pound you into an uncaring jaded disinterested unloveable husk of a being after too many emotional scars from losing loved ones, too much of seeing humanity make the same mistakes, and too much watching the knowledge you gained turned irrelevant.

Or, life will beat into you an uncanny ability to converse and relate to others, even if fleetingly.

Watch The Man from Earth.

Time gets shorter. I’ve already experienced this going from my teens to my twenties and into my thirties. I can remember entire weeks of my childhood. By the time I was in my mid twenties, days and weeks blurred together. Now it’s like months go by and I don’t even notice.

People talk about it more as they get older. Eventually when you enter your 80s and 90s, it’s like entire decades can come and go. So imagine when you’re immortal. If you’ve been alive for 100,000 years, that’s longer than writing has been around. Entire civilizations will have come and went.

But from your perspective, it’s all a blur. Entire genealogies were experienced, yet those people barely registered in your mind. If you had a favorite food, maybe the recipe disappears when you went four centuries without eating it. Jokes and fashions you’re familiar with are completely alien to everyone else. Are you even capable of noticing when things change at that point?

There’s also the question of how human are you? Everything and everyone would seem inconsequential. Would you even be able to socialize with others, or would you be completely sociopathic? That’s if you don’t hurt anyone and get tossed in a jail cell. What happens if you spend a few centuries in prison? Fight in multiple wars? Would you even feel the slightest discomfort when you kill someone?

Discovering the upper limits to what the human mind can retain and just constantly forgetting all the shit you used to find important.

I don’t think you’d remember a break up from hundreds of years ago, let alone be upset about it.

Basically all of the time you’re alive will be after the heat death of the universe, where you will be floating in space, with nothing to do, nothing to see, nothing to experience. Complete darkness, complete silence, in a complete vacuum, for eternity. Every other particle in the universe is forever out of your reach. You know that you will have nothing forever. You will never see, hear, or touch anything again, for all of time, which will never end. The trillions of years that preceded your float through the void fade into a distant memory as you outlive twice as much time, four times as much, a trillion-trillion times as much, and infinitely more.

On one hand, you have eternity to come to grips with everything you’ve done. On the other hand, it might take eternity to come to grips with everything you’ve done.

Seeing all of your friends and family die, knowing you’ll never stop missing them.

Having the perspective of centuries. Seeing society make the same mistakes over and over again because they forget, but you never do. It would drive me mad. Already does, considering I have the ability to, and have, read history. I just imagine living it over and over to be tedious.

You know how the curse of pet ownership is that you will almost certainly outlive them?

That, but with everyone you love

I’ll say no one can truly know. Unless you are yourself immortal