User blog comment:TheBlueSlug/Ending/@comment-9339850-20130725094714/@comment-9339850-20130725101937

Well because over the course of 15 novels, Issei and Rias have grown to care about the other girls as family, and Issei has become a person none of the girls want to lose in their life and vice versa. Rias sympathizes with the other girls, understands their feelings for Issei and realizes that having the harem isnt the worst thing; As i put in the other post, she doesnt mind the harem because she knows that she is the most important to Issei and will always come back to her in the end.