What are regrets?

"What you regretted all this time was the fact that you weren't able to do anything to stop her from going to the dark path, Rioux." Rain spoke up gently, upon noticing Ryo lost in his own thoughts, or should he say, reminiscence of his own past. 

"So, it wasn't to stop the war?" 

Rain chuckled, hearing Ryo's response. "War? There have been countless of wars, arguments, and clashes in the world. Civilizations after civilization were wiped out just like nothing but a spec of dust, even though they used to have a mighty, glorious past. But those made the world what it is right now. Do you think it's regrettable enough for us to bring forth every lost soul?" Rain got up, and started to pace back and forth with a calm and steady posture, raising slight tapping sounds with the shoes. "There is another reason why you feel so contented right now, although."