Love is clearly depicted in the first act as fickle through Romeo's own fickleness. When we first meet him, we learn that he is madly in love with a woman named Rosaline. He is so madly in love, that his heartbreak over her due to her rejection has completely overpowered him. As we learn from his father, Romeo has been seen staying out all night long, night after night, and sleeping through the day. He has also been seen each morning at dawn, standing under a grove of trees in a certain part of town, crying, as we see in Lord Montague's lines:
Many a morning hath he there been seen,
With tears augmenting the fresh morning's dew,
Adding to clouds more clouds with deep sighs. (I.i.127-29)
In addition, when Benvolio tries to dissuade him from continuing to think of Rosaline and to consider other women instead, Romeo's response is to argue that he can't possibly think of other women because there can't possibly be a woman more fair than Rosaline.
However, Romeo disproves his own argument and proves to be very fickle when he falls in love at first sight with Juliet. Not only does he prove that he was not truly in love with Rosaline, he disproves his belief that he can never find a woman more beautiful than Rosaline. We especially see his change of heart and fickleness described in his lines, "Did my heart love till now? Forswear it, sight! / For I ne'er saw true beauty till this night" (I.v.54-55). Not only do these lines perfectly portray Romeo's sudden change of heart, they also portray exactly why is fickle--he mistakes real love for merely physical attraction.