Loud munching sounds came from across the table of your booth.

The source? Aso.

Aso's team won a swimming competition at your school and he wanted to celebrate with you, and you alone by going to a diner to get food and dessert.

Aso was wolfing down the burger as if he'd never had something so delicious, which was a lie.

He's had burgers multiple times, but not with you.

Maybe he was trying to impress you in some sort of weird athletic way you've never heard of?