Something just struck me about the zombie genre. Even if you've already established a working civilization, if you go for an excursion into infested areas, all it takes is a little bad luck to die. Beyond the fear of being a part of the undying horde, there is the fear that you might not make it back. Any and all trips run the risk of failure. Any promises you make to make it back are hollow, because the threat of dying is always present. In a lot of ways, I guess it's like Final Destination. For all you try to fight fate, in the end, you're still just one bad move away from dying. I think the sheer force that random chance has in their lives is where a lot of the horror comes from: one slip up, and they're gone, forever.
Just my two cents though.
Interesting points... but, that's life, isn't it? Every time you get into a car and onto the highway, you are in a similar place - one mistake, be it yours or someone else's, and you might die. Raise a toddler and look away for a second and they put something in their mouth and choke on it. In every day life, we have plenty automated security mechanisms (#) - but leave the stove on, slip on the sidewalk or in the bathtub, lean an inch too far over your balcony, even a sudden event like a heart attack (or some other sudden failure within the body) all go to show how fragile life is, how we are indeed 'one slip up' away from dying, constantly. (# routine, balance, automated coordinated locomotor system, laws and restrictions, signs, conditioning, hygiene... - and even technology, both to warn us as well as to self-check in case of failure, neglect or mishandling, e.g. fuses)
A few thousand years ago, and even in some areas today, humans evolutionary hardly any different from us physically, are/were constantly threatened by whatever nature threw at them - going out for a hunt while easily turning into someone else's prey, settling near a river for water supply while having no protection against occuring floods...
Constantly worrying about "if I mess up / something goes wrong, I might die" would massively decrease quality of life, though. So we go about and trust in the above mentioned security measures, or in case of more dangerous living situations focus on what we might gain in exchange for some (arguably reasonable) risk. Otherwise we might stagnate or even degenerate - and that seems to be against human nature. We "want" things. We want more than to survive - we want to (or we want our beloved to) "live" to the fullest potential available. And for that we may even risk our own lives, all with the optimism that we will succeed - because without that positive attitude we might just curl up and wither away....
The point being, that it's not only about survival, but also about living. Of course those apocalypse survivours could hide inside a closed enviroment, hoping their resources will last indefinitetly. But resources may need replenishment. And if not that, one may or may not strive for diversity or even some sort of bettering of the status quo; so they go out, scavange and salvage and if possible conquer additional area to both increase quality of life as well as acquire an increased sense of security (with the monsters being pushed further and further away) - even if that means risking ones very life in the process.
So, in a nutshell, it's less about seeking adventure or some secret death wish, but about "quality [of life] over quantity [of lifetime, in case things go wrong]".
In that sense, the zombie and alien genres have long moved away from splatter action moster horror and into sociological character studies. The focus is less about the monsters and surviving or even beating them, but (re-)learning to live despite their looming presence. ...so, basically what Yuki represents, I'd think.
