Kevin had been exhausted only moments before seeing Dean's note, but as soon as he'd read it, he was no longer the slightest bit tired. A mission, they had a mission! Within seconds, Kevin was dying of curiosity to know what it was, and unable to wait much longer.
His body still remembered that he'd been tired, though, which was why he'd been in the tent lying down. Instead of running out to flail madly in Dean's general direction (wherever Dean was), he wrote back, asking him to come to the tent. He hoped that fifteen minutes was a generous estimate, and that his friends would be as excited as he was. Waiting for fifteen minutes seemed like torture right now.