you know why it is so sad?
Just imagine the little vampire boy as someone who fall in love,
he want to grow the given plant that need sunshine - something he know he will not be able get
but he get it nonetheless - for his life as a price, because he love the person who gave him the plant and growing the plant was proof of his love.
we are just the same, when we fall in love, we often have to get something we knew we are not capable of, but we get it nonetheless - for a greater price than it should - because we want the one we love feel our love for him / her
