In Act I, why specifically is Romeo uspet/sad?

Not only does
she not love him,  "she hath Dian's wit." Diana was the goddess of chastity. This girl
Rosaline has commited to remain chaste, meaning a virgin. The reader might even be led to
believe that this is a girl who has decided to become a nun. This is highly probable with the
Catholic overtones you see throughout the text.

So, it's not that she's just
not interested right now, or that...