Everyone thought she was a class A bitch. But did they even understand?
Of course they didn't. Alice didn't know what it was like to be human, it's not like she had to live through the pain of losing everything. Bella, Bella was just naïve and plain; she would never understand. She got her fairytale ending. Rosalie didn't.
Esme might have understood but even she didn't discuss it with Rosalie, which only made things worse.
The guys were guys, they obviously wouldn't understand. Emmett, though he tried, couldn't take away her pain.
She never chose this. No-one would. Being the eternally damned was hardly anyone's dream. It got tiresome and it was beyond meaningless.
Even vampires didn't live forever; they died, just not as soon as they would if they were human. They were only granted more time, at the expense of being monsters. Killers.
And Rosalie knew she would rather be an old withered woman waiting for death to take her to the grave after living her life to the full, rather than being a woman eternally young and beautiful waiting for her existence to be put to an end.
So she shut everyone out. She shut them out because she never wanted any of this. She built up a wall that she let no one in; because she never wanted immortality or beauty unmatched by another's.
She wanted to be left to die on that fateful night. Too bad we never get what we want. Or Rosalie would be lying peacefully in her grave instead of counting down the days until the end of her pitiful existence.