Okay, so, i was recently watching Brawny Buck's A Buck to the Face on The Crystalling and i noticed something Twilight says that may give logical reason why Starlight honestly thought Cutie Marks were what took her friend from her and why she felt justified in getting revenge on Twilight by breaking their friendship by stopping the Rainboom.

When Twilight tells Starlight she's found out that Sunburst lives in the Crystal Empire, she uses the exact words "the first pony you ever cared about" to describe him.

Now, this is an interesting thing to call another, is it not? Sunburst was the ONLY pony Starlight ever cared about before? What about her parents? Grandparents? Aunts and Uncles? Cousins?

Well, the answer is one of two, the latter surprisingly simple and has been touched upon in the fandom.

option 1: Starlight cared nothing for her family at all and focused all her time on studying until Sunburst came along and began opening her heart, only for it to close again once he moved away.

option 2 (and makes far more sense, in my personal opinion): Starlight is an orphan and didn't get along with anypony else at the orphanage, not the carers, nor the other fillies and colts. However, when she met Sunburst, for the first time in her life, she found somepony she could connect to and they became close friends. But, when Sunburst got his Cutie Mark and moved away, it made Starlight feel like his Cutie Mark was what took the only pony she'd ever let into her heart out of her life.

This led her to be reserved and keep to herself more than ever for the fear of losing another friend to a Cutie Mark related reason and her reasons for creating to Cutie Un-Marking Spell and why she wanted to keep such a firm grip on her village. With her in control, she didn't risk losing anymore friends ever again, not realizing that was doing the opposite.

This is also why, when Twilight and the others took that away from her, she felt justified in taking away their friendship, because she felt she no had no one again.

I feel number 2 is the most likely.

It makes a disturbing and rather depressing amount of sense, when you think about it. it would explain why nopony ever tried to help Starlight break out of that thought process as a kid. If she'd had parents, they would've tried to comfort her, likely even would've made sure she keep in regular contact with Sunburst via letters, or maybe vacations to Canterlot for a visit.

However, if she was living in an orphanage where she already didn't get along with anypony, nopony would notice any change in her behavior from what they were used to prior Sunburst, so wouldn't have tried to help her out, having already given up long before Sunburst became her friend.

Just something I couldn't help thinking about after noticing Twilight's word choice in that part of the episode.

What about you all? Does this make sense to you?

Pretty much sums it up

6015688
She could just be nuts. Or her family could have been negligent/abusive/senile.

They might have been nice too, and Starlight is simply a bad egg.

6015688 I think the reason people have a problem with it is because she never tried to get another friend and let one event destroy her life. Even Starlight just metioning that cutie marks took away all her friends would probably remove a lot of peoples problems with her. While I'm not here to say that would have been a good idea or not it would have made her seem less petty and people wouldn't have a giant conformation bias agaisnt her.