Be careful what you wish for, or you may get it. Be careful what you wish for, or you may regret it.
Suppose an immortality device actually works, and you use it and become immortal. Irreversibly...
What are your prospects?
Inevitably, you will grow old and frail (think of the knight Indiana Jones found guarding the holy grail). Your immune system will be all worn out, so you'll catch every disease to be caught.
As your medical record grows to fill a room of its own, someone will finally figure out that you cannot die. You will try to hide that fact, but they will find out sooner or later.
Immediately, you will be treasured. What a perfect test subject! They will lock you up and test every new drug on you. After all, what harm is done if you cannot die? After they have destroyed your liver, heart, lungs, kidneys, and the rest of your organs, it will finally dawn on them that they are hurting you after all.
They will apologize, pin a medal on your chest, and let you go.
By then, unfortunately, all your friends will have died, and you will have hard time making new ones. The pace of society will have changed so much, you will not be able to adapt easily.
But adapt you will, eventually. Alas, people will have evolved into a different species: They will become more subtle. They will stop using speech for communication. Perhaps they will develop telepathy and use it exclusively. Will you?
Then, some day, perhaps millions of years from now, mankind will go the way of the dinosaur. You will stay on a planet with no one like you.
Not so, you may be saying. There will be others, others who have also used the immortality device!
Guess what! After a couple of million of years, you will be avoiding them like a plague. And they will be avoiding you. Besides, there will be no one left to push your rusty wheel chair, no way to recharge the batteries on your nuclear powered one.
But you will adapt. Then, one day, there will be no other life left on the planet. With no food to eat, you will be hungry.
Eventually, all water will have evaporated. Not only will you be hungry, you will be thirsty.
The planet will have turned into a rock. You will be gasping for air, but there will be none.
After the planet has turned into dust, you will be floating weightlessly in the open space. Your muscles will have atrophied from the lack of exercise.
You will be hungry. You will be thristy. You will be suffocating. You will be lonely. You will be bored. And you will never die.
So, once again: Be careful what you wish for, or you may get it.