Let's play pretend for a moment. Okay,you are a waiter/waitress at a cafe. You see this person that comes in regularly.

You think, Oh probably to eat this again.

You were right once you took his order. After he/she had eaten you gave him the receipt. He puts down the money and you look find a white piece of paper.

You decided to see it later and hand the money with the receipt to your manager.

Later he gives it back to you. You look at it and you see it's a love note.

Everyday the stranger comes,but the person doesn't leave a white note. 

Wait...so who is it?

That little scenerio explains Lee Sungmin's and Cho Kyuhyun's life.