"The Lost Thing" is a story about a boy who finds a strange creature at the beach. He tries to locate its owner or rightful home, but everyone around him dismisses the creature's presence. Despite the lack of support, the boy feels sorry for the creature and sets out on a journey to find where it truly belongs.
At the end of the video the protagonist said “I see that sort of thing less and less these days though. Maybe there aren’t many lost things around anymore. Or maybe I’ve just stopped noticing them. Too busy doing other stuff, I guess.” and by that line I guessed that the lost thing is a metaphor for youth and dreams.
By growing over time, as a result I think that all the dreams and expectations we had when we were young gradually fade away.