From what I gathered the boy borrows the love of someone, but since it wasn't achieved by himself, that happiness that he felt just slowly fades away as if wasn't ever true, and even though he were experienced with his happyless, sorrowful, sad life, once he experienced what a heartfull life, happyness was, he just couldn't bear tô come back to his old, loveless, happyless, empty hearted self.
It says that the girl at the end, the love full heart donator, is full of love and pure hearted, since she cries for someone she doesn't know, showing sympathy.
Ai no donā