Shino had a stalker, and you were that stalker. You have sent him countless love letters, and he read every one. To you, they were symbols of your love. To him, they were off-putting and obsessive. He wasn't used to this attention! He was creeped out, mainly because they were anonymous.

so, he wrote you a letter back, and put it in the spot where you put your letters to him.

and the letter had his scent..