Is Philosophical Pessimism based on a... mood? I thought it was in this thread, but I can't find the post to respond to now, so I'll just reply at the end here.
Someone said that if life was intrinsically worth living, then we would be satisfied with just existing. From experience, I know that that is sometimes the case, but it occurs to me it's also easy to illustrate that it could be the case, for those who have never experienced it first hand.
Imagine someone has an unlimited slow drip of some drug that gave them a constant nice little high, maybe an opiate of some kind, such that, in the absence of all other stimuli, just sitting or laying around somewhere, they just feel this slightly warm happy feeling and are just content doing nothing, happy just to lay there and feel good from the drug. They will eventually get hungry, and the hunger pains will eventually overpower the pleasure of the drug, and drive them to go get some food, but once their appetite is sated they'll go back to just having that nice slightly warm happy feeling and being content doing whatever. Likewise, all their other bodily needs. Meanwhile, other activities can still bring them even more pleasure, so they have motive to go and do other pleasurable things when they can. But so long as their basic needs are met, they don't need constant new stimulation, because they've always got this constant little bit of pleasure feeding into them from this little drug drip.
All such drugs mimic substances that the body produces naturally; that's why we have receptors that react to them the way that our brains do. Your body is capable of producing "drugs" from inside itself that can put you in exactly such a state, where merely existing is enjoyable in and of itself, not in some catatonic drugged-out way but in a way that you're happy just continuing to be alive and don't need for unlimited entertainment or something. Some people's bodies produce more of these "drugs", others produce less of them, and both genetics and life experiences can affect how much the body makes.
I think most children are born in a state where just existing is inherently enjoyable, though of course there are pains that can overpower that, and other pleasures that can be had beyond that. I remember being that way when I was young, and I think I probably somehow managed to stay that way much later into life than many other people, and also had plentiful experiences of that taken up to 11, the so-called "mystical" or "religious" experiences of meaningful wonder and bliss about nothing in particular. Life experiences, the pains and pleasures thereof, had largely beaten that out of me, and I think do the same for most other people, seemingly much faster for many others than for me.
So if a child can be raised in a way that will spare them that roller-coaster of a life that leaves them without the joy of just existing -- that ontophilia -- that they were born with, then it will have been worth bringing them into the world. They will probably still die eventually, but they'll probably look back and say it was worth it. It may be very hard to spare anyone such a roller coaster of pains and pleasures and preserve that ontophilia, maybe even as hard as keeping them alive forever, but "very hard" doesn't mean "impossible". It is therefore an individual, case-by-case calculation about whether any particular life is more probably worth bringing into the world than not. It may still be the case that in many, many cases it reasonable is not worth it -- I don't have kids for precisely the reason that I don't think I can give them a life worth living -- but you can't make a blanket proclamation that it is always better not to exist than to exist.