Why is Romeo sad in Act 1, scene 1?

Romeo's sadness is first noticed by his parents and his
best friend, Benvolio.  They have noticed Romeo likes to keep himself locked in his
room, in the dark, and often goes out walking by himself.  In fact, just that morning,
Benvolio saw Romeo out walking, but they didn't talk to each other.  Romeo tells
Benevolio he is sad because the girl he loves, Roaslind, doesn't love him back. 
Though Romeo'sconversation with Benevolio, the reader comes to understand that Romeo is
more upset that Rosalind wishes to remain a virgin.