Tybalt, the kinsman to old Capulet, Hath sent a letter to his father's house.(II.IV.7-8)
Benvolio MercutioStreets of Verona (Noon)
Romeo will answer it(II.IV.10)
A challenge, on my life.(II.IV.9)
Benvolio and Mercution discuss Tybalt's letter to Romeo challenging him to a duel. (Revenge)
Nay, he will answer the letter's master, how he dares, be dared.(II.IV.12)
Any man that can write may answer a letter(II.IV.10-11)
The reader can infer the motives of Tybalt's challenge due to a scene earlier in act 1. In it, he is outraged to find out Romeo's attendance to Lord Capulet's party, and vows to pay him back.
This relates to the motif of revenge, because Tybalt saw Romeo's attendance at Lord Capulet's party as a strike to the Capulet's pride, and challenging Romeo to a duel is his way of getting revenge on the Montagues for it.
This relates to the overall theme of opposing forces because this duel relates to the Capulet-Montague feud. Shakespeare is correlating the existence of opposing forces to revenge; the tensions and grudges of opposing groups can culminate into revenge over petty things, like attending parties.