Josh and Arnold deserve each other
I hope those two are stuck with one another for all eternity.
Arnold will keep whining to Josh about every thing that upsets him, and Josh will keep teasing Arnold, making subtle digs at him, and playing emotional games with him whilst pretending he's having an innocent bit of fun.
Finally, at some stage, Josh will say something jokingly (but actually secretly in full seriousness) that makes Arnold snap, and Arnold will murder Josh in his sleep. Arnold will then be arrested, stand trial, be declared mentally incompetent, and then be locked up back in a mental asylum again for the rest of his life.
A perfect series finale.