Human evolution, that would be the easiest answer. Generations after generations of forgetting values and the importance of the world they live in, of taking things for granted and developing a more comfortable and easier way of live. Of worrying only about themselves and not even thinking about what world they are leaving for their children and yet, when things go wrong and they suffer, they complain and blame anyone but themselves.
[Yoh chuckles an humorless laugh.]
You know, for the humans in my world, the world is like a plant your mother used to take care of, always watering it and talking with it, making sure it was always healthy and perfect... But you never saw her do those things because you were at school or playing around, all you know is that the plant is always there, that it gives pretty flowers from time to time, and that you don't need to do anything for it to do so. And then your mother dies and the plant is left at your care but because you were always busy you never learned how to take care of it, you don't even know that it needs to be taken care of, and thus you ignore it, expecting it to remain as always and give flowers when spring comes. And for a long time it does, although slowly it starts to take longer for the flowers to come and they last less time, and the green leaves look less green and bright, but it's a very gradual change, you eventually die and the plant is still there and your kids do the same as you did, which is nothing but ignore it, and so do your grandkids until eventually the plant finally dies. And by then... The people in charge of it don't even know why it's there or if it was important so they just throw it to the garbage and go out to get another plant and only then they will realize that the plant was special because it was the only one they could ever have.
They didn't let die the plant on purpose, but they take it for granted and forget that it needs to be taken care of, that there are things you mustn't do and things you must do... It's no one's fault, it's everybody's fault even those who have long since died.
[Action]
[Yoh chuckles an humorless laugh.]
You know, for the humans in my world, the world is like a plant your mother used to take care of, always watering it and talking with it, making sure it was always healthy and perfect... But you never saw her do those things because you were at school or playing around, all you know is that the plant is always there, that it gives pretty flowers from time to time, and that you don't need to do anything for it to do so. And then your mother dies and the plant is left at your care but because you were always busy you never learned how to take care of it, you don't even know that it needs to be taken care of, and thus you ignore it, expecting it to remain as always and give flowers when spring comes. And for a long time it does, although slowly it starts to take longer for the flowers to come and they last less time, and the green leaves look less green and bright, but it's a very gradual change, you eventually die and the plant is still there and your kids do the same as you did, which is nothing but ignore it, and so do your grandkids until eventually the plant finally dies. And by then... The people in charge of it don't even know why it's there or if it was important so they just throw it to the garbage and go out to get another plant and only then they will realize that the plant was special because it was the only one they could ever have.
They didn't let die the plant on purpose, but they take it for granted and forget that it needs to be taken care of, that there are things you mustn't do and things you must do... It's no one's fault, it's everybody's fault even those who have long since died.