Since like everyone in my class is posting this already, I might as well do so too.
The lost thing represented something that is no longer available in the boys life. I agree with many other members of my class, and I think the lost thing represented childhood as well. The lost thing is unique and nonexistent in another persons' mind as every childhood is different. Though childhood is an important part of human life, people do end up forgetting bits and pieces of their memories. As the boy grows up, he stops seeing many 'lost things' and that may represent the fact that he's slowly moving on and forgetting parts of his life.
