Where's the betrayal?

There's still a piece I'm missing. Where's the betrayal? Was it the bet about whether Miyata-kun would show up? Yet, it was Hoshizawa Senpai who was at the train station alone and waiting. Miyata-kun talks about losing. Does he think his own feelings in high school were more than Senpai's? And he now says he was just being teased then, so he must think Senpai didn't actually care at all.