They say that when two people are in love, their hearts beats as one. Scientists found that a couple's breathing patterns and heart rates would match up after sitting close to each other. They didn't even need to touch or talk to each other when this happen. Hinata was confused by his sudden change of perception towards Kageyama. He's looking for the answer if two hearts can really beat as one. After he realized what he truly feels, he found the answer to his question.