Stiles used the time he had ordering a banana split (because he needed more objects to assault orally, clearly) to put together an answer in his head. He decided the mostly straightforward approach would be best. "The other you had lost someone important to them thanks to the alpha pack and he and I aren't exactly the type of friends that hugged so I hugged you instead."

Because you were the type of friend he hugged, Derek. Isn't that nice?