Why?

He was sweet and all understanding. He lended her a shoulder to cry on when she felt the need to fall apart. He was there for her when she needed someone.
Somehow she was only telling people from Hufflepuff. She had told Susan and now Justin. Maybe it were the qualities of the House that she liked: just and loyal. She definitely needed a friend. Especially now that she had told Parvati everything. From the begin to the end.
She loved Justin for the fact that stood by her. She loved him like a brother, she liked him. Then why did she suddenly have the desire to sleep with him?