Because Running From Your Problems is Health...
When the Pack starts pushing Stiles out, he makes a choice to leave Beacon Hills. He plans to stay with his cousins, Sam and Dean for a week, until he can figure out what the hell he's doing with his life. Before he leaves, he makes a stop at Chris Argent's place.
Read the tags before you read the book instead of complaining about the content thanks. ;) (I'm not saying this to be rude, I've just had bad experiences with another book where people didn't read the tags then complained about pairings in it, and my choices in where I went with it.)