The reason behind Blueblood’s treatment of Rarity. Sadness turning to anger.
 
______________________  
from duplicate post:
 
>You adore someone  
>Want them to notice you  
>Did something awesome  
>Failed  
>Come in A (as a name)  
>Did the same thing you did  
>They awe-ing A  
>Your heart cracked  
>Repeat  
>fail  
>Fail  
>FAIL  

 
This is the same thing applied to Bluebloods situation as my little headcanon. Blueblood always tries to get Celestias attention. Though she wouldnt give the amount of attention he wanted, he would passed it off as shes tired or busy, and keeps on trying. But when a new celestial pupil, Twilight Sparkle stays in the castle, Blueblood noticed that Celestia always spends her time with Twilight. Giving the filly more attention than him. He feels a twinge of sadness and jealousy but pushes it away with a thought of Twilight will leave soon. But days turns to weeks to months to years, she stills here. Celestia ever so attentive of her antics. The grown Blueblood turns cold and hate the purple mare. When Twilight was send to Ponyville for the Summer Sun Celebration, he thought its over and that he could finally have Celestias attention but no. Instead, the alicorn keeps on talking about how smart and talented Twilight is and that he should be like her. The pains for all those years finally get to him and shatters his heart. He turns into a rude snob and cold-heart towards Twilight, Celestia and anypony that has to do with them, including Rarity at the Galloping Gala.