Are you talking about when we first meet her in the play, or when she has just been banished by her uncle? If it is the latter you are refering to, you can find the answer by reviewing Act I scene 3. Examining Rosalind at the beginning of this scene reveals that she is very depressed, in spite of her cousin's attempts to cheer her up and make her smile. Note how she bemoans the struggles and toil of the world when she says: "O, how full of briers is this working-day world!" The entrance of her uncle with the news of her banishment of course does not help matters much. However, as the scene progresses, and Celia manages to talk to Rosalind and bring her around, Rosalind's attitude seems to change dramatically, as she and Celia begin to look forward to their exile and journey into the forest, once they have considered the various precautions they need to take as young single women. Note Celia's last speech, for example:
Now go we in content
To liberty and not to banishment.
There is a sense in which this "exile" actually liberates Rosalind and Celia, and we can imagine that Rosalind is now much happier about leaving court as she can do it with her beloved cousin and also look for her father.
Before going to the forest Rosalind was fully in love with Orlando. She was just thinking about him. When duke said her to leave the court her attitude became rebellious and she revolted.She asked a specific reason for her banishment....