Love always finds a way
It would be an understatement to say that Stiles and Derek went through a lot. They’ve both lost a lot, had to endure a lot, tried to find their peace with women. But in the end it always comes back to them. Only them.
Why not just finally admit that they might as well make it work as long as they can. With the lives they have, running from one supernatural being to the next and any day could be the end, they don’t want to waste any more time. Just because it might not last doesn’t mean it can’t be something good.