This is not how Stiles wanted to come back. Him coming back isn't about him and his ex-boyfriend. It's supposed to be because Scott and Allison are getting married. It's supposed to be a fun reunion and not the kind that maybe his friends are still hoping for him, something that Stiles doesn't know what to think about. Sure, Derek may be his one great love, the one that got away as well, but that didn't mean that Stiles hasn't moved on. Derek clearly has and Stiles has too. Stiles being back doesn't change any of that. It doesn't change the fact that it has been three years and that it's over.