He always wondered why every time you see a chance, you'll immediately do it with no hesitation. And that is exactly what you've been doing, often leaning on his chest.

He doesn't understand why, and what are you probably implying by wanting to hear his heartbeat. But he never said no even if you and him were just friends, on the other hand he agreed just for his attachment towards you.

One day in the middle of a bright sky, you both are talking things about academy. Not so long later, you asked to lean on his chest. Now, he decided to ask you question since he never did. "Why so? Do you have clear reason?"