Seamus was a little surprised by her statement, but tried not to show it. If Lav didn't want to date anyone, he supposed that was her business - he only hoped, for her sake, that she wasn't still pining after Weasley. He'd bet all his non-existent fortune that when their missing housemates came back from wherever they were, Ron and Hermione would finally have gotten their heads out of their respective arses and done something about how mad they were for each other. And he put a lot of energy into hoping that that it would be 'when' and not 'if.'
But he knew Lav had really cared for Ron, and as her friend, had spent a fair bit of last year slightly hacked off at his dormmate for jerking her around. He just didn't want her still to be waiting for someone whose heart was elsewhere. She deserved someone who'd focus his attention on her and how great she is.
He smiled at her. "Well, that's their loss then, yeah?" He fell into step with her as she steered them towards the Great Hall, his grin growing wider as she seemed to regain some of her good spirits. "Ah, come now, Lav. I was more of a prat than you were a brat - I'm sure I should eat at least twice as much as you."