Hirota is the typical guy who doesn't know what he has till it's lost. He was embarrassed to be seen with her when he got older and got his girl snatched even if she was being manipulated. In fact, that's what Miwako wanted to do was take away from him the very reason Hirota denied her in the beginning.
On some "I'll take what you hold dear for not being seduced by me" type of shit, sole reason as to why she chose him instead of Souichi