In summary i think this whole show is about hopelessness and how your attitude to it should be. So out of the 4 human characters in this show, 2 have a passion that we witness them being at their peak in their "career" and the almost immediate downfall thereafter. Kanazawa is a cartographer and he had just finished mapping out the entire first level to the city they are all in. His next mission is to simply move to the next level and then keep mapping, but as he gets on the elevator to the next level he loses his lifes work in just an instant and almost kills himself trying to save it and when he is caught by yuu and chii he tells them to just let go and let him die. They obviously dont and they all move on to the next level, where Kanazawa comes to terms with his lifes work being lost and simply sets out to do what he came to the second level to do, Create a map of it. Even though he had just lost everything that gave his life meaning he decided to keep trudging on. Ishii also has an incident similar to this in that her lifes work is building a working plane and flying to the next city over. when chi and yuu finally see her off she soars in the air and is flying for a brief moment before her plane breaks mid air and she is seen in a parachute heading down to the first level. she says something along the lines of "so there was no hope afterall... all those years... toiling away alone.. gone in an instant... but now that ive failed... i feel so relaxed" like kanazawa she comes to terms with her lifes work being destroyed and she even feels better that a weight has been lifted off her shoulders. After all they have ever worked for becomes destroyed they both come to terms with hopelessness and just stop caring. While we see this transformation mid show i feel that yuu had come to terms with this long ago hence her personality. When chii and yuu are stuck in the middle of nowhere because the kettenkrad was broke down chii is trying to fix it while yuu is just lying on a slab of concrete chewing on a piece of metal telling chii that "its hopeless chii just come to terms with hopelessness already." telling chii that there is no reason for her to worry about fixing the mini tank because there is nothing they CAN do to fix it. Why burden yourself with worry if you dont even have the ability to lift that burden? I believe that this is the reason why throughout the show yuu is always care free and happy. In a literal wasteland there isnt as much to worry about as there is in a society. You need the basics, and if the couple needs them, then well the best they can do is look for them. If they still cant find the things they need after trying then thats just how things were meant to be, no reason to get yourself in a fluster over something you cant control. I also think this is why while she show is in a dark and dreary setting, with death surrounding them at almost all times, it dosent have too much of a sad tone to it. The intro and outro are both upbeat and happy, and the general mood of the show is melancholic at worst. I think this show teaches a great lesson, and that is to just relax, stop worrying, and if something bad happens to you, which it will, you need to realize that bad things happen all the time and you dwelling on the past will only hurt you. in the end you decide your mood and if you want to be happy then you can be happy.

footnote i also wanted to talk about how in a world where food and water are running out very fast all of the characters are simply riding out the last of the remains of civilization, doing what they want to do in the mean time. But i forgot what my whole shpeil was going to be about that so yea. Also if you cant understand a point that i was trying to get across in that wall of text that is because i have a hard time putting the word sewage that comes from my brain into comprehensive words.