User blog comment:B214/Volume 22 spoilers/@comment-27083298-20160610145130/@comment-24.113.252.213-20160610190201

What I remember is Grayfia saying that it was normal for a devil woman to already be married and have a child by the time she was Rias' age, so she should start seriously considering the state of her love life. With that as an excuse, Rias was forced to undergo the Trials at the Ruins of Connection with Issei.

This was basically the Gremory Family's way of saying "if you're going to be in love with someone like Issei, then hurry up and make progress in your relationship!"