With this specific soliloquy in mind, it would most make sense to argue that Juliet was infatuated with Romeo. This particular soliloquy occurs in Act III, scene 2 after the Nurse led Juliet to believe that Romeo was killed. Once the loose ends are tied, it's clarified that Tybalt, Juliet's cousin, was in fact killed by none other than Romeo. Grief stricken, Juliet sends the Nurse to fetch Romeo so that they can celebrate their recent marriage. In a brief moment, Juliet has transformed from an excited bride to a "widow" and a young lady seeking the Nurse's advice to a woman capable of making decisions on her own.
The soliloquy you must refer to is full of oxymoronic expressions to describe Romeo. An oxymoron is when two seemingly contradictory terms coexist. I've taken the soliloquy you provided and bolded examples of oxymorons.
O serpent heart, hid with a flowering face!
Did ever dragon keep so fair a cave?
Beautiful tyrant! fiend angelical!
Dove-feather'd raven! wolvish-ravening lamb!
Despised substance of divinest show!
Just opposite to what thou justly seem'st,
A damned saint, an honourable villain!
O nature, what hadst thou to do in hell,
When thou didst bower the spirit of a fiend
In moral paradise of such sweet flesh?
Was ever book containing such vile matter
So fairly bound? O that deceit should dwell
In such a gorgeous palace!
In this short monologue, Juliet uses about 10 oxymorons in order to express her sorrow, disbelief, disdain, and love. For example, she recognizes that Romeo's good looks may have caused her to overlook his evil heart: "O serpent heart, hid with a flowering face!" Furthermore, the word "tyrant" has a negative connotation, yet she pairs it with the word "beautiful," just as the word "fiend" has a negative connotation, yet she pairs it with the word "angelical." Throughout this soliloquy, her emotions for Romeo are quite conflicted, which could bring us closer to your thesis.
When a person is infatuated with someone, he or she will experience an intense passion for the person; however, that passion is normally short-lived.
In the case of Juliet, we can argue that her passion for Romeo was intense due to the fact that they married so quickly. We could also argue that her passion for Romeo is intense using the diction in this very monologue. What forces that infatuation to be short-lived, however, is the fact that Romeo killed her cousin Tybalt, thus, making Juliet feel conflicted between honoring her new husband or her family.
Here are some basic and more advanced thesis statements you could work with:
In the monologue 'O serpent heart, hid with a flowering face,' William Shakespeare highlights Juliet's infatuation with Romeo through oxymoronic phrases.
In the monologue 'O serpent heart, hid with a flowering face,' William Shakespeare reveals Juliet's "true love" for Romeo as just infatuation through conflicting, oxymoronic phrases.
In the monologue 'O serpent heart, hid with a flowering face,' through the use of oxymoronic phrases, William Shakespeare shows that Juliet's infatuation with Romeo is causing her to feel conflicted between her duties as a wife and a daughter.