I think it's about a guy who really liked a girl, and a girl who was crazy for that guy. But both the girl and the guy had a deep, dark secret they weren't willing to share with each other. This secret caused them to drift away and finally coming to the conclusion where they decide to part. However, they are still both madly in love with each other, causing them to say "Will you still miss me? 'Yes, I'll miss you.' Do you love me? 'Yes, I love you.'" They're battle to keep loving each other is depicted as "this union". They desperately tried to keep their love alive, but in the end, it was never about love.