User blog comment:B214/Talk About What You Want! 2/@comment-74.77.52.225-20140716184143/@comment-24098614-20140716202753

The problems that Rias were ones that she couldn't solve, and one reason that Issei was so valuable for her in fact Sona was jealous of this because it is a rare thing for a servant like Issei to solve such distressing problems like she had, and Sona couldn't do a thing about it because as a HIgh Rank Devil her hands were tied up. Sona wanted to help RIas very badly with her problems because they are rivals but they are good friends, and she thanked Issei profusely for the help that she gave her remember Masters are only as good as their servants, and Issei went beyond the call of duty for her.

This is one reason why Rias cares for Issei so much on top of all the other reasons that she fell in love with him even thoughhe is a pervert his heart is in the right place, and he tries to help his friends without wanting anything back.