Previously, in Act II, Scene IV, we witness Benvolio informing Mercutio that Tybalt sent Romeo a letter. Mercutio speculates that Tybalt is challenging Romeo to a dual, saying, "A challenge, on my life," meaning "I'll swear, or bet, that Tybalt is challenging Romeo" (II.iv.8). Mercutio's mood may be in part a reflection of his mood in learning about Tybalt's letter. Mercutio may be feeling defiant and ready to defy, or resist Tybalt's threats. Hence, Mercutio's reason for being out on the streets that day is that he hopes to challenge Tybalt right back.
One clue telling us that Mercutio may be feeling defiant in this scene is that when peace-loving Benvolio begs Mercutio to get off the street, Mercutio responds by ironically chastising Benvolio's temper. He accuses Benvolio of being like a man who lays down his sword in a bar but draws it again, ready to kill, by the time he has had his second drink (III.i.5-10). He even calls Benvolio "as hot a jack in thy mood as any in Italy," meaning as hot tempered as any man in Italy (III.i.12). The irony is that we see Benvolio always attempt to make peace. It is Mercutio who has the temper. Since Mercutio is ironically attempting to cover up his own temper by using Benvolio as a scapegoat, we can assume that Mercutio is feeling angry right now, or defiant, and is out on the streets simply because he wants to challenge Tybalt.
The mood in the beginning of Act 3 Scene 1 for Mercutio is very sarcastic, then turns to challenging and anger.