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

The disappointment of experience winning lifetime supply of something but that would eventually turn into a lie

Wage slaving never stops

If the ultra rich find you out, you can expect lab-rat life, at least until all modern systems collapse. Death is the only thing those suckers fear, because regardless of their net worth, it comes for all, even if late. They would do anything to find out your trick

Loneliness. I think being immortal would show someone what true Loneliness is

i think that experiencing all the things & people i care about would be the worst of it.

either that or seeing us repeat history over and over again as a society complete with all of the indifferent cruelties it entails and studying it, but ignoring it anyways

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.

Everyone else in your life that isn’t immoral (if you’re the only one who is) dies eventually, so every time you make a friend or start a family, you do so knowing that you will have to watch them all die someday.

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.

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.

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

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

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.