The last lines....

"This is a story about an uncertain future
A story about trusting someone to share your fears with
A story about making each other's lives fuller"

All in all I think this is the pivotal point of the whole story. And this is why the authors chose this ending. Even the characters realised that their future is uncertain, that they might not be happy along the way. But they chose to trust each other and try again, because in this cruel, relentless world, they are the only ones who can fill each other's otherwise empty, aimless lives.