Asklemmy

If it's just you being immortal, loss of all family, and friends, and loss of new friends, rinse repeat forever. Eventualdieyoull watch society collapse and regrow (possibly), and the planet will die. Immortality is forever after all. Then you're left alone on a deserted dead planet. Electronics you have will eventually break and fade away to time. The sun will grow and die off, and it'll burn because you're immortal but still stuck on a planet that'll get enveloped, eventually. Living forever would be terrible unless it was forever until you died of something physical, just not age and illness.

All the people trying to dissect you for science.

If everyone gets to be immortal, imagine never being able to get rid of dictators. Putin's 600th won election.

People in the future wouldn't be allowed to have children, Earth will be filled to the brim with very old people and very few new ideas.

As OP mentioned, a lot of replies focus on loss, that friends will inevitably die and objects will break........ we already face that reality with regular life! That's hardly a downside of immortality itself.

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

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.

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.

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

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.

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

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.

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.

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.

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.