User blog comment:B214/Question and Answers 3/@comment-4547267-20140707220256/@comment-1359322-20140708001421

Well there are actually a few reasons.
 * 1) Venelana's scolding.
 * 2) Issei accepting her love and becoming her boyfriend.
 * 3) Like Jop said, Issei's "death".
 * 4) A form of respect, since Issei accepted Rias for what she is, Rias has to do the same.
 * 5) Maturity on Rias' part, in the past she would never do that. She would just be selfish and forced others to accept her arrangements.