Secret admirer
He was very good at comforting girls. He had comforted Ginny when she had lost all hope of ever getting together with Harry. He had comforted Cho when she mistook Harry's friendship with Hermione for love.
Now he was comforting Parvati. Another that once dated Harry.
Parvati needed to be comforted. People constantly accused her of being a lesbian, constantly taunting her about it. Even her own twin sister had joined them.
Myabe he truly loved Parvati; then why was he getting all horny at the sight of this other girl. The girl wasn't even pretty: it was Millicent Bulstrode!