It would be in poor taste to say that Derek had a weakness for dead parents and guilt, but as soon as Stiles' anger faded into quiet pain, he own anger dissolved. His claws and teeth retracted, and he exhaled, sharply.
"I know that feeling," he said, softly. The human's pain called to him, and before he realized what he was doing he'd moved across the room and had reached out, settling his hand on the boy's shoulder. "I'm sorry. I didn't know. About your mother."