@Tk3997  
How would Celestia know that the human world had a larger or more advanced civilization? She hadn’t gone exploring.

@Sjogre  
Probably for the some reason some people argue attempting to contact alien life is a really bad idea. Namely that life doesn’t rise to a dominate position with being capable of violence, valuing itself over others, and being naturally vary of possible rivals. Sure maybe a contact would go okay, or maybe it could end up in a war or invasion, or at the very least bring tech and ideas that could seriously destabilize the nation.
 
Or if we assume Equestria isn’t that large (not unreasonable given the cast zipping all over it) and doesn’t have a huge population a much larger civilization could easily destroy it INCIDENTALLY via assimilation. Really there are any number of good reasons not to go galloping off to expose yourself to a far larger and more advanced civilization.

@Background Pony  
She wanted to reiterate a previous lesson about “humility” by showing her the mirror. The mirror can apparently show hidden things as well as act as a portal.

It seems a tad irresponsible and short-sighted of Celestia to show Sunset the mirror and just immediately shut her down when she tries to learn more about it.
 
Her reaction to Sunset seems to be “I know I showed you this mysterious and interesting thing, but if you follow your scholarly instincts and ask about it you can go fuck yourself.”
 
At least explaining her reasoning, if not exactly her reasons, for keeping the answer from her could’ve prevented Sunset from going down the path she did.
 
I haven’t read the whole thing, this is based on the few scans I’ve seen, but it seems that Celestia acted too harshly before Sunset crossed the line and lead Sunset to crossing said line.
 
Probably why Celestia seems so kind and understanding with Twilight; she learned not to be too harsh in her admonishments lest she drive her down a dark path like she did with Sunset.

@Evowizard25  
She said she knew little of the Empire….Which is odd, since I doubt it was that far away.
 
First off, it was that far away. It still is. It was in a frozen wasteland, far to the north of most of modern Equestria, and probably even more distant a thousand years ago. In modern times, it’s still far enough away to be outside of Equestria proper’s weather control program.
 
Second off, the Crystal Empire wasn’t part of Equestria back then. It was a small country in the middle of the great white north, as far as she was concerned. Until Sombra started messing with the emotion broadcast stuff, Celestia didn’t have a reason to go up there.
 
The mirror, by contrast, was kept inside of the Canterlot Castle. With little in the way of guards, and unlike the statue of Discord, it wasn’t in an inert state.

HA! Sunset is the cutest villain ever!
 
Also Celestia’s reasoning for keeping the mirror secret and not having a single clue about what’s on the other side….Really flawed.
 
I mean, what’s with Celestia not knowing that much about stuff she’s involved with. First it was the Crystal Empire where she and her sister apparently defeated Sombra. She said she knew little of the Empire….Which is odd, since I doubt it was that far away.
 
And now this mirror…Didn’t she think to explore? Or did she already do so, but just keeps silent because….  

@Star Cavalcade  
Of course I do. Friendship is magic ;)  
My joke just failed to follow through, oh well :P

@ColdhardSilver  
I know, but you know she and her friends would go along with that.  
I mean, it’s a world of talking animals. Fluttershy would be camped outside that every single day looking for a way in.

@Itsthinking  
Except that Celestia doesn’t know that much about it. She hadn’t explored the world on the other side of the mirror before prohibiting it.

@Star Cavalcade  
I was kidding, Star :P
 
@Itsthinking  
Also can you imagine what would happen if humans found out about magic!?

@Sjogre  
I still think Celestia does it for the sake of the human world. I personally do not believe that human encounters with an intelligent and sapient species would be full of anything but anxiousness and worry.

@ColdhardSilver  
Why would the ponies think that? They’ve made contact with a bunch of sapient species, so the idea of mass panic from first contact is unlikely to occur to them.

@ColdhardSilver  
Her friends?

@Star Cavalcade  
True, but who would believe Pinkie Pie on either side? jk :P

@ColdhardSilver  
Isn’t that exactly how it ended in the movie?  
Human Pinkie wanted to make it through to the pony dimension, how long before some other people try it?  
How soon until SG1 is set up on the human side and they send MacGyver over?

@Sjogre  
Sake of the ponies and the humans?  
I mean it’s like the “what if aliens landed?” scenario.  
There would be mass hysteria on both sides once news got out.

@Itsthinking  
Exploration of the Everfree is not restricted. Most ponies might be afraid of the unknown, but explorers are still held in high respect, and they do exist. Ponies will voluntarily go into dangerous and mysterious situations.
 
Why would Celestia prohibit exploration of the mirror?

@Background Human  
… Aside from the complete lack of implication that such a thing exists, we’ve seen the ponies infringe on buffalo territory without a second though. Buffalo don’t have magic.
 
Knowledge of another world existing, complete with another sapient species, doesn’t mean much to a world that already has numerous sapient species. Certain facets of the other world might be alarming… Except Celestia hasn’t explored enough to see anything.

@Background Human  
I get the impression that someone stumbled through it, Celestia tried to chain it up in the dungeon, it escaped and she’s trying to cover it all up.
 
Lyra will uncover the truth however, bet her comfortable ass on it.

@Sjogre  
It’s the Equestrian Prime Directive: no interference in the development of pre-magical civilizations.

Equestira’s Pangea?

@Sjogre  
Ponies are inherently scared of things that don’t follow their rules. It’s the herbivore mindset.
 
Ponies aren’t like humans. If humans had something like the Everfree Forest, it would have been tireless explored.

@ColdhardSilver  
Which is a terrible reason to avoid. Actually, not knowing about it is a good reason to study it.
 
In a safe place, sure, but study it all the same.

@Sjogre  
Princess Celestia: “You will soon know more about that world than even I know” (From EqG)  
So maybe fear of the unknown?

@Sjogre  
IDK.  
Celestia Reasons?