Cozy Glow From Equestria? · 12:01am Jan 11th, 2020
Okay so why does Cozy Glow act so naive? We've seen Equestrian and even pony villains use bullying and intimidation, under the Princess of Friendships nose none the less. If she is a power hungry filly who's good at strategy, why play the part of a cute ignorant filly? Why bother with such a charade, especially if she could garner more respect as the student who knows more about friendship than anyone, unless the ruse was familiar to her because she has done it before? But how does being naive and sweet get her control over others? It is hardly a fast track to power but it is a fast track to attention and adoration.
So maybe Cozy grew up bored and disillusioned about her place in society, maybe she didn't understand what her mark means or maybe she did know she but didn't see how it was particularly useful or relevant to her. Still she decides to reach out to others anyway and as a result, discovers that she is very good at anticipating other ponies needs and providing for them. In the process makes her self the most loved filly in her local. She is after all helpful and productive, everyone has a story of how helpful Cozy is, and everyone is on good terms with her, and she likes it. She fills good, she feels powerful like she has a handle on her destiny for the first time in her life.
This works because her town is like Ponyville everyone knows each other as neighbors but they are not really neighborly. People are enough apart from one another that Cozy fills a void in the same way that Pinkie Pie does. But then the Friendship Journal is published and unlike most of Equestria her town gets it. They start reaching out to help their neighbors and their friends and Cozy is no longer unique in what she does. She of course does damage control, but in spite of her best efforts she is never again the local darling. What makes her special has been taken away from her. Cozy finds herself needing to replace that sense of fulfillment she lost. Then she learns Twilight has opened her School of Friendship and she decides to attend in order to learn what makes Twilight's version of friendship so special and refine it to gain back her popularity and sense of self worth.
She makes friends with and studies Twilight so can learn her secrets. She manipulates the CMC (who are the sisters of the Elements of Harmony and thus should know more about friendship than anyone else not directly connected to the school) into helping in such a way so that they become her case studies, allowing her to study friendship in action so she can replicate it. Making friends with them by getting them into the school of friendship is the final exam she sets for herself, so she will know that she's succeeded in perfecting her understanding of it.
However the longer she stays at the school the more frustrated she gets. Twilight only got her position not because of not her friendship skills but because of her magic; her only friends are ones of convince, where as for Cozy, friendship has been a life long pursuit, she believes that she deserves the power and adoration that comes with Twilight's title a lot more than Twilight does but eventually concludes that as long as Twilight's around and has support from her magic and friends she'll never have it. So she starts plotting to regain her popularity by removing her biggest obstacles to the adoration she craves, Twilight, the friends her will support her no matter what, and the magic that gave her authority in the first place.
She reaches out to Tirek because he has the necessary knowledge she needs to drain Equestria of it's magic. He provides with the underlying mechanics of how his draining spell works (for reasons of his own). She researches ways to make it work by herself on a grand scale using the books in Twilight's library (which she has access to as Twilight's assistant). There she runs into the Young Six while researching. Needing them to leave her so she can get back to work (and possibly to avoid them finding anything incriminating) she manipulates them into doubting their friendship so they will leave her alone, (and latter does it again to ensure they won't tattle on her to Twilight).
In the process she discovers the catacombs; a perfect place to enact her plan. She creates a fake Key of Unfettered Entry for Twilight's group and sends them off to Tartarus. It is easy enough to imagine Cozy luring Starlight down to the catacombs with a promise of a quite place to work and then 'discovering' the energy storm. From their it would be easy to lure her into the pull of the storm by pretending to be in danger. The rest of the story we know.
Now this is a much more plausible take - I'm not sure why you'd abandon it for such a wild theory as your other one. It even ties into the show's fundamental themes.
And why would she need to be a runaway?
I'll take that bit out. As far as why I abandoned it their are two reasons.
One Jim Miller says unequivocally that Cozy's parents are no longer around and the last time he said something inane and I ignored it, they doubled down and made it canon, so I wasn't excited to come up with a theory only to be shot out of the water by such a obvious oversight.
As for the other; this theory really only works if we assume Cozy has a natural talent for connecting with or manipulating others, and after examining her character again, I am no longer convinced that is the case.