His own punch had been a shock to the rational part of Seamus' mind, Dean's retaliation even more so. He hadn't even thought to defend himself, and the blows, though not the full extent of his friend's strength, were strong enough to rattle his head. For a moment, it felt like Seamus had had the sense knocked into him - that he had realised this fight was ridiculous and the two of them simply needed to talk to one another - but a moment later a surge of adrenaline pulsed through his veins like hot fire and Seamus lost all sense of anything except his desire to physically release his anger.
He threw himself bodily at Dean, his rage making his fight style stupid and clumsy. Instead of aiming for his friend's face, he wound up a punch towards at Dean's gut. He would wind him and then take out his anger with leisure as Dean struggled to recover.