>I disagree with the fundamental starting point of the argument, I don't think any of the points will convince me.
You've never read a text that proposes something you don't agree with? Seems pretty limited. Not to mention, if everyone acted like you do, this book would have been read by 0 people. It's not meant to be pleasant, you know...
>You cannot sweepingly say
It's quite an advanced argument. He doesn't "just say it", it's tackled from many points of view.
>Experience is fundamentally subjective
No. You feel hungry, I feel hungry, that's a negative for both of us. You can rationalize it for yourself in that it's worth it, but biologically, that experience can't be anything but negative.
Refer to the chart linked here anyhow, the argument isn't so much about how much negativity there is in one's life, but that any negativity is bad, and we can avoid making lives that experience it.
>to a large extent people decide for themselves whether their life is "good" or "bad"
Yes, but we aren't reliable sources for it. You go ask people in India without jobs, shitting on the street, no house, hungry - "life is a miracle, I'm grateful for it", but you wouldn't conceive if you knew that's how your kid is gonna end up, would you? Same for a kid with down syndrome, or similar illness.
>Why are suffering and pain bad things that humans shouldn't experience?
What do you mean, "why is pain bad"? Because physiologically we're programmed to sense it as a negative event. In moral philosophy, if inflicting pain isn't isn't considered bad, I don't know what is.
>Something that exists can decide to cease to do so, but something that does not exist can, obviously, never choose to do so.
This would mean something if it were simple, but it isn't - suicide is a long, difficult, painful process, that damages not only the individually (prior to pulling the trigger), but the people around him/her, too. We aren't robots. People all around the world are infinitely miserable, and still don't kill themselves - why wouldn't you want to avoid the creation of those beings?
By the way, literally all you mention is better argued against in the book, in case you're interested.