Explain the dramatic irony the friar expresses in lines 39-40

Friar thinks Romeo has been upset because of something that bothers him.

But where unbruisèd youth with unstuffed brain
Doth couch his limbs, there golden sleep doth reign.
Therefore thy earliness doth me assure

The audience knows what Romeo has been up to and his anxieties about being in love with Juliet, a Capulet.