When Stiles finds himself turned into a werefox and a alpha at the same time he doesn't know what to do so he lets the pack believe that he died. It was working out rather well, Stiles had a small pack that included the guy he formally wanted to kill aka Jackson, a smart born wolf, and the adorable puppy Liam. He had ties to large packs that helped him out and he was happy. Of course he missed his old pack but he believed that there was no going back. But everyone knows how these things go, the past always comes back.
So after two years of assuming Stiles was dead when Derek finds out Stiles is in fact alive he feels a little more than betrayed.
Summary is shit sorry.