User blog comment:B214/Question and Answers 2/@comment-4547267-20140403182948/@comment-16281343-20140403191652

Because he knows Asia is very important to Issei. Even before Rias fell in love with Issei (Volume 2), she saw how much Issei cared for Asia (Volume 1).

Remember in the start of Volume 2, she made Asia live with Issei, kinda shipping Asia with Issei.