Why is romeo sad in act 1 scene 1?

In the opening lines of Act 1 Scene 1, Benvolio observes that Romeo appears dejected and gloomy. However, at this point in the play, Benvolio does not explicitly state or explore the exact reason behind Romeo's sadness.

It is not until later in the act, particularly in Scene 4, that we begin to understand more about Romeo's state of mind. In Scene 4, we discover that Romeo is deeply enamored with a young woman named Rosaline, but his love seems unrequited or unattainable. Romeo's unfulfilled desire and emotional longing for Rosaline cause him much distress and heart-wrenching sorrow, leading to his melancholy mood in the beginning of the play.