Madeline didn't need anyone to remind her of the pain it would bring. Yes, it would be painful, but there is a difference between getting pierced by a thorn and Cassian's fang sinking straight into her skin.
The difference was the pleasure, there's no pleasure in getting pierced by a thorn, or maybe, the only reason why she doesn't consider the pain was because the one with the fangs was Cassian? Whatever the case might be, Madeline knew she had to do this to help him receive his strength, so regardless of the pain and pleasure, she was doing it because he needed it.
Madeline's heartbeat leapt from her chest to her throat when Cassian knelt in front of her. He didn't even bother to respond to her question and just went down on his knees. His red beaming eyes glanced at Madeline, still withholding himself in case he sensed any hesitation or fear from her body language, but to his awe, she looked rather determined as she stared straight into his eyes.
