To me it is mostly that life is boring. At some point you have seen everything interesting and anything you see hence are just fitting into the same basic patterns. Discussions, tv shows, books etc. Living is still better than not living, but every time you experience something new the things left to experience becomes smaller. A few romance movies and you have seen all of them, a few action movies etc, or even psychology papers or math papers etc.

You haven't seen all of them, but the more you see the less value you get from seeing another. So we can conclude that living for an eternity wouldn't be particularly more interesting than living for 100 years, as 100 years is more than enough to see many things in each category. You might not become a world expert in everything, but experiencing the things worth experiencing in the categories doesn't take particularly long to do and after that life is just padding.

The main reason to live now is that there are new interesting things coming out still, since computers are still young. That adds new things over the years. But interesting progress isn't really the norm, it isn't like physics or math has gotten any more interesting over my lifetime and I expect computers to get there before I die and then there isn't much left.

Edit: For example, everything we produce today is based on the same math and physics my grandfather learned in college. There has been no practical progress since then there. Main thing since is new tools enabled by computers that are more precise or automate things, that is interesting but the discoveries enabled by more automation/precise tools will run out at some point.

I'm not too familiar with Zen, but questions like these are often meant to be nonsense questions with no answer. They push you to think harder and harder until you realize the futility of doing so, at which point you've found the "real" answer.

Previous poster either got this subtly wrong, or phrased it badly, or belongs to a tradition that I'm not familiar with, but a good story on this is the parable of the poisoned arrow: Someone gets shot by an arrow and his friend tells him they have to get to a hospital - the person shot starts worrying about what kind of poison it is, who shot him and why, if the hospital they are going to is any good, etc, etc - really he needs to go and get it treated RIGHT NOW and nothing else matters - he can answer all those questions after he's treated.

And I wouldn't say the zen koans are unanswerable, you can think of them like riddles that force you to think a different way than you normally do.