Because he's still alive. I'm not sure that's enough evidence though; it started me thinking about whether I found my life worth living, and I just don't know. (It's a bad subject to get me thinking about, I'll try and stop it ASAP after this post).

Right now, I'm a fat middle-aged translator who has to translate boring technical stuff. OTOH, I do have a sweet husband and a lovely cat, and the people who pay me for those translations must think they add some value to the world.

In the 56 years leading up to that, do the positives outweigh the negatives, taking into account my tendency to remember the negatives (especially the embarrassing ones) much better? I can't tell, but if someone were proposing to create another one of me, to live the life I have lived so far, I'd say: "Why on earth would you want to do that?".

That isn't to say I'm ready to commit suicide though. I do have my husband and cat, I have a holiday to look forward to, and death is probably a very nasty experience, especially a DIY one. [EDIT: fixed typo]

Why does it seem like Stuart considers his life worth living?

Revealed preference.