User blog comment:B214/Question and Answers 3/@comment-117.222.3.157-20140718132854/@comment-1359322-20140718140629

That's for comedic purpose only. The only Devils that can match a serious Sirzechs is Ajuka and Rizevim.

Sirzechs is usually goofing around so Grayfia who is always serious tends to be very strict with him. Grayfia is in charge of Sirzechs' personal, professional and public life. And to make things worst, Sirzechs is usually unable to answer back to Grayfia because Grayfia is the one who is right. Like in Volume 13, Sirzechs abandoned his duties and stalked on Issei as Satan Red.