Maybe she just didn't want to be alone... Maybe she had some primitive need for partnership. Some primitive survival mechanism that urged not to be alone? And Draco fascinated her. There was a certain pull, something that had always been there. Maybe because he was the perfect embodiment of a prejudiced pureblood? And she wanted to therefore prove him wrong as the quintessential muggleborn? Maybe it was the excitement of their rivalry in school? Or merely the guilt of having failed to save him from Azkaban? Maybe it was as simple as that she sensed the same conflict, the same darkness in him as she had. Not evil, but not purely light either. Whichever it was, he was the only one she could imagine with her on this journey. So, she went and got him for herself.