Say you won't let go is telling us a love story that everyone hopes and wishes for. The song starts with how the two lovers met. James says that he was in a very dark low place at that time. But love is such a powerful emotion that it took away all the negativity from his life and filled his life with happiness. The song basically tells their whole life story, how they fell in love immediately. The sweetest thing about this is the line "I wanna stay with you until we're grey and old". This shows that he loves her enough to spend the rest of his life with her. He wants to have a family with her and live a happy, complete life.