User blog comment:B214/High School DxD EX/@comment-109.147.36.194-20150727164406/@comment-3965848-20150728063210

Well they're traveling to the point when Issei was still oblivious to Rias so that could be a reason.

Although we can't be sure if he's really mad, maybe Ikusu is just saying he's a lot more serious compared to his father.

It's better to keep an open mind.