Harry thinks every death is a horrible tragedy. So, wouldn't he want to bring back everyone in the past as well? Make their deaths "not happen"? So he goes back in time to Atlantis, to arrange a self consistent history where no one has in fact died, but only seemed to die, much as he suggested Dumbledore do for Hermione's death.

One of the first lessons was ComedTea, and thinking about causality going backward in time. There's people like Harry and BDumbledore thinking about history as a story. There's the ridiculous levels of foreshadowing we see. History looks like a story because it is one, written by Harry. There was more powerful magic in the past because future Harry took those powers back in time with him.

There's even the foreshadowing of Harry going back in time to play a trick on himself.

"Ssalutations from Sslytherin to Sslytherin."