Why wouldn’t living forever be just like any other scenario where a good thing is multiplied by infinity? The novelty would wear off just like chocolate or sex. Things are “good” because they are scarce. Never-ending anything would become a burden.
If I get tired of eating chocolate or having sex it is because I want to do something else. I can’t really ‘do’ anything besides living (death isn’t me doing something because I no longer exist). We are also programmed to only want a certain amount of sex and chocolate, but we are for the most part programmed to want life as long as we can get it. Life also has a lot more options than more specific ‘good’ things. I always have too many things I want to do in a day. It is hard to conceive of waking up one day and thinking I was bored of life or just wanted to stop existing. I have to imagine pretty dire circumstances.
Then again, I haven’t lived tens of thousands of years, I might very well get bored and decide I was done with life. But I still would like the option to live as long as I want, just in case I don’t.
The difference is that life, given an infinite amount of time also has an infinite amount of options for things one can do. There are enough things to do forever, the only question is whether the specific individual will keep thinking of things that they want to do. The crux of our disagreement seems to be that you think people would get bored with literally everything if they lived long enough and I think that most people would find something worthwhile in the infinite possibilities. But neither of us have lived very long (cosmically speaking) so it is difficult to really know how we will feel if we live to be 500, 5000, or 5 million years old.
Returns are diminishing for one activity, but there are infinite possibilities of activities one might do in infinite time. I don’t think diminishing returns applies to everything you could do at once. But again, I don’t know, maybe continued existence would eventually become unpleasant. That’s a possibility I’m not ignoring, but just because its a possibility doesn’t mean we shouldn’t strive to have immortality be an option.