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.

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.

Sooner or later, you will get trapped somewhere forever. Over the course of an infinite lifespan, the odds that a building collapses on you or a tunnel caves in on you basically become 100%. Someday, you will fall into the hole that you will stay in until the sun explodes, and then you will drift in the void until the heat death of the universe.

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.

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.

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.

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

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?

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.

Getting trapped under something for a few thousand years.

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.

Cancer. So much goddamn cancer. It doesn’t matter what kind of immortality you have, you WILL get cancer. Repeatedly. Over and over. Forever.

I had a really nice washing machine. Then it broke. The manufacturer was dissolved 25 years ago.

I had a really nice cast iron pan. Then it fractured. Modern cast iron pans aren’t smooth.

I had a really nice car. Then a part broke. Replacement parts haven’t been available for 50 years.

I had a really nice flip phone. It was made by Nokia so it still works. People think it’s weird that I use a flip phone.

I had a really nice peace and quiet. Then someone invented ambulances. Now I cower in the corner of my bedroom hiding from manmade horrors beyond my comprehension.

Boredom after some period of time, you will have some everything there is to do.