This song tells the story of how they sacrifice for their beloved. They remembered the memories when they were together, but it just a beautiful memory that can not be repeated. They tell how they feel when they had to let the girl go. At the end, the lyrics "having you by my side, you've lost your smile" and "you can only be happy once you leave my side". It's like that the girl wasn't happy or something, so they let her go so she could be happy again. So she could smile again. To me, they do a big sacrifice. They don't care if they had to suffer for seeing their beloved to smile again.