When Romeo first meets Juliet, he is wearing a mask, which becomes symbolic of how deceived Juliet feels by Romeo later in the play. When Juliet first learns that Romeo has killed her cousin Tybalt, Juliet feels like she has married a murderer instead of a hero; she feels like she has married the devil instead of an angle, as we see in her speech full of oxymorons:
Beautiful tyrant! fiend angelical!
Dove-feather'd raven! wolvish-ravening lamb!
Despised substance of divinest show!
Just opposite to what thou justly seem'st. (III.ii)
Since Juliet feels deceived by Romeo, we quickly see that the mask he wore when meeting her symbolizes his upcoming deception and represents the theme of disguise and deception.
Later, Juliet does some deceiving of her own. In order to prevent her marriage to Paris, with the help of Friar Laurence, Juliet deceives her family by faking her death. However, the plan goes awry and thinking that she is truly dead, Romeo kills himself which consequentially leads to even Juliet's own real death. As Friar Laurence phrases it, "A greater power than we can contradict / Hath thwarted our intents" (V.iii). Hence, in this part of the story, the theme of deception is used to tell us that deception has consequences.
Ultimately, Shakespeare uses the theme of disguise and deception to tell us that all things may not be as they seem, especially violent, passionate love, and that disguise and deception have lasting consequences.