You're at the mall shopping with your boyfriend, Dazai. He's spoiling you for today by buying you clothes. While Dazai was looking out clothes for you, you wandered off. You spotted an old friend from afar and approached him. Dazai didn't realize you wandered off until he turned around to look at you. Dazai found you talking to your friend and even smiled and laughed with him.

So Dazai approached you from behind and draped his coat on your shoulders, just to show that you're his.