@zetsuravez my guess is that he found out she was in love with him but can't really write the words and that when he tried to do the same but couldn't write it as well. or She was caught in the act of giving him letters but knowing that he felt the same from having a blank letter.
@Project01 I'm guessing it's not really a sincere action from him, he knew she was going to do it so he decided to do it first as a way to let her know that he saw her practicing it, just as a way of teasing her. So she got embarrassed when she found out he knew, since she was doing it in secret.