Asklemmy

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 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, even if you just go on forever.

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.

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.

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.

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

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.

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

That, but with everyone you love

Having to keep track of that evil snail.

https://www.reddit.com/r/AskReddit/comments/5ipinn/you_and_a_super_intelligent_snail_both_get_1/

Being eaten by sea anemones, tuna, sharks, swordfish, sea turtles, penguins, and other jellyfish.

https://en.wikipedia.org/wiki/Turritopsis_dohrnii#Predation

If other people are also immortal, the awkwardness of all of them eventually becoming your exes

One of my books features an immortal protagonist and I've as such thought about this quite a bit. More than the answers already provided here, what I found interesting as a writer was the balance I needed to find between making an immortal detached from mortal values while still being engaging to mortal readers.

Said as a pithy question, if you can outlive everyone's decisions and mistakes, what would it take to make you do anything at all?

That old person feeling of no longer being with "it", and what's "it" now being strange and scary probably compounds over the centuries.

Given a long enough time frame, the vast majority of an immortal life would be spent buried beneath something or floating in the void of space. Think about it, you outlast planets and stars. When those go dark, but you don't die...nothing to do but float in space.

You might counter that with, "well yeah, but eventually I'd find other sentient life forms and/or people again.” And sure, maybe, but that wouldn't last as long as you...and then you're just alone floating in space again, for the vast majority of your life. The only thing to look forward to, since you will outlast everything, is the end of time itself.

Without getting into the heat death of the universe and all that, I can think of something that happens much, much sooner. I'm only middle aged and I already don't like where the world is going. Can you imagine being centuries, or eons past the era you identified with? Can you imagine how insufferable young people and old people alike would seem when you have centuries worth of life experience and wisdom? Can you imagine a horde of little edge lords on the internet confidently yet incorrectly telling you about the signing of the Declaration of Independence, when you were there when it was signed?

Knowing the answer to some of history's biggest mysteries, because you were there, but being unable to speak about them because, 1, that would expose you, 2, nobody would believe you either way because nobody expects you to be THAT old.

Also, it is already frustrating seeing kids being dismissive or denying events that you yourself have lived. Imagine being thousands of years old and seeing so much shit, but those events are rarely retold, forgotten, or straight up denied by conspiracies or future governments that won't admit their fault on it.

Having to constantly find new hiding places for the blood chalice, and keeping up with all the latest scanning methods so you can develop countermeasures. Your secret is never truly safe.

Depends on the type of immorality. Do you continue to age? If no, what age do you stop? Eventually the universe will die. So what happens to you then?

It might be fun for a while. Maybe even a long while. But that fun will be gone in an instant compared to the trillions and trillions of years you will float in a dark dying universe of nothing.

Not being able to kill yourself.

immortality doesn't guarantee perpetual health, you're alive, but so broken and sick you wish you could die, but you can't

Being asked your birthdate in order to view a game on Steam, and the year dropdown not going back far enough.

Getting imprisoned for thousands of years unable to get out.

The eventual heat death of the universe would be painful

Just depression in general. I don't want to live one lifetime, let alone never being able to die.

If you're immortal in a body that isn't broken then that might be a different story, but you'd still grow to love people only to have to lose them and go through that pain over and over.

Based on your question, you might dig the book “Boat of a Million Years.” The author put quite a bit of thought into just that.

https://en.m.wikipedia.org/wiki/The_Boat_of_a_Million_Years

The Sun will eventually fry all life on Earth and boil off the water & atmosphere. Eventually the Sun will die out completely, leaving you on a cold, dark rock.

As we get older, our perception of time speeds up. An immortal would easily lose track of time after just two human lifetimes, causing an immortal to suffer from dementia-like symptoms where they expect one date but find themselves habitually late. And since time doesn't mean the same thing as us to an immortal, they would eventually become disconnected from the world around them and be unable to reintegrate. They wouldn't be able to maintain friendships, relationships, mortgages, payments, etc. They would be surrounded by people but forever alone.

Government Bureaucracy.

Renewing a driver's licence or passport. The individual looking at your application will see the date of birth and raise a red flag.