Because whatever Neville was about to say to him was bound to be infuriating. It was also bound to be negatively slanted towards Hermione. Neville would never have refused Harry's inclination to summon her so pointedly if it wasn't, as he'd done barely ten minutes ago, when he came to fetch Harry to break this news to him. He was about to tell Harry something that he thought Hermione ought not to know, which was pretty naive of Neville ... for whatever it was, the first thing Harry intended to do was tell Hermione all about it.