The main idea the story tries to tell you, I think, is that anyone has a place where they belong. "The lost thing" represents outcasts in society. It personally struck me as a symbol of disabled people. The awkward organization of "the lost thing"'s body (so hard to identify it, btw.) particularly made me think of physically disabled people. The comment the narrator, or the main character makes at the end of the movie, I think, implies that we tend to be set in the order of day-to-day life naturally and comfortably, so that we won't take risks of involving ourselves with things we are unfamiliar with, afraid of, or uncomfortable with.
I suppose that you can taste a little irony in the story. When he found "the lost thing", the man recalls "Nobody else seemed to notice it was there. They were all too busy doing other stuff, I guess." But in the ending, he says "maybe, I've just stopped noticing them.....too busy doing other stuff, I guess." Thus, the protagonist who once found "a lost thing" turns out to be "too busy doing other stuff" to find another one like the people on the beach were, which is unexpected? (I still don't get the idea of irony completely. I welcome you guys pointing out my misunderstanding.)
In other news, my dad came to see me all the way from Aomori to Tokyo. I'm having a great weekend with him. I hope you're having a good weekend, too. See you tomorrow.
