"もし他のすべてが滅びて彼が残っても私はまだ存在し続けるだろうしもし他のすべてが残って彼が消滅したなら宇宙は途方もない見知らぬものに変わり私はその一部にはなれないだろう"

Simplified Meaning:

This quote talks about the deep connection someone feels with a specific person. It means that if everything else in the world disappeared but that one person stayed, they would still feel like they exist and have a purpose. However, if everything else remained but that important person was gone, the world would feel strange and unfamiliar to them, like they don't belong in it anymore. Think of a child who feels safe and happy as long as their parent is with them, even if they are in a new and scary place. But if the parent is not there, even their own home can feel lonely and scary. The advice here is to cherish the people who make you feel whole and connected to the world. For example, spend quality time with those you love and let them know how important they are to you. This can bring comfort and a sense of belonging, no matter what else changes in your life.