What's the meaning of Dad's fantastical story about the Sixth Borough, when a boy keeps love in a can?

Oskar's final memories of his father involve a fantastical story of the Sixth Borough, which requires faith rather than fact for its validity. When Oskar sets out to solve the key mystery, he is undertaking a search for an objective answer. However, through his journey, he realizes that he is actually seeking an objective answer to the tragedy of his father's death. Ultimately, Oskar grows to discover that not everything in life has a scientific answer, and that we are happier when we can accept the value of intangible, emotional experience as at least equal to that of scientific fact.