User blog comment:B214/Volume 22 spoilers/@comment-50.139.232.230-20161022060406/@comment-9595671-20161024105804

If we go by the logic that they love Issei, then everyone but Gasper and Kiba would have wanted to join them. Another reason aside form history is that Rias saved Akeno from her clan, Kiba when he was dying from poisoning, Gasper from vampire hunters and gave Koneko a home.

With the others, Issei saved Asia from Raynare, Xenovia would have died if Issei's efforts to help Kiba didn't give her and Irina more aid, Ravel I think fell for him due to his determination and the fact he didn't take it too far with Riser after he lost, and rescued Rossweisse from Euclid when he tried kidnapping her.

So it basically boils down to who saved them in the end except for Ravel, but maybe we can consider Issei beating Riser (and helping him recover maybe) saved Ravel in a different sense. It did seem like she became a lot sweeter when she and Issei met again, cause I don't remember seeing her sweet side when she first appeared.