User blog comment:B214/Volume 21 info & spoilers/@comment-95.90.224.210-20160602194640/@comment-27876048-20160603160236

In regards to Issei's intelligence, he has shown to be extremely perceptive and "thinking outside the box" so to speak. He has gotten much smarter over the course of the series or at least knows more then he initially did.

Like B214 said, he is just showing proper respect to people who deserve it. But I do agree even though I think its mostly playful teasing on everyone's part it does get annoying how everyone puts Issei down but they also give him alot of respect at the same time. If High School DxD Ex is any indicator Issei will gain way more confidence in himself, hell I would even go as far as to say he may even get a little cocky