I think the lost thing not represent something exactly.
The lost thing means things or feelings which are not worthy for us anymore,
and it varies from person to person. For me, they are like small shells.
By the way,
the world the lost thing finally went was so colorful and seemed to happy and
comfortable, on the contrary, the human’s world was gloomy and discolored. In
my opinion, that means humans, especially adults have lost many things as they
adapted to the society, but actually, the lost things are much more valuable than
the things they cling to.
I think the message of this movie is “Haven’t you
forgotten something important, or the fact you forget something? Do you still have
what you truly wanted to protect, or threw them away without knowing?”.
