Benvolio speaks the line, "[a]nd I will make thee think thy swan a crow," when he is trying to persuade Romeo to forget about Rosaline (I.ii.91). In the previous scene, after seeing how heartbroken Romeo is over Rosaline's rejection, Benvolio begs Romeo to "[b]e rul'd by [him]: forget to think of her," but Romeo refuses to listen (I.i.227). Benvolio next hears that Capulet is giving a ball and that Rosaline will be present, so Benvolio thinks it will be an excellent chance for him to introduce Romeo to some new women, which will help him take his mind off of Rosaline. In the speech containing the line in question, Benvolio is trying to convince Romeo to crash the ball with him for the very sake of introducing Romeo to different women, as we see in Benvolio's lines:
Go thither, and with unattainted eye
Compare her face with some that I shall show,
And I will make thee think thy swan a crow. (I.ii.89-91)
Not only is it evident that Benvolio is trying to distract Romeo with different women, it is also very evident that Benvolio actually thinks very poorly of Romeo's choice in Rosaline, especially in comparison with other women in Verona. Benvolio's opinion of Rosaline is evident in his metaphor, "I will make thee think thy swan a crow." Swans are very elegant, beautiful birds with bright, gleaming white feathers; they are also not as common as other birds. Crows, on the other hand, are large, black, ugly, make awful sounds, and are very, very common. Therefore, Benvolio is comparing Rosaline to crows in his metaphor, which says he does not think very highly of either her beauty or of Romeo's choice in her.
A theme Benvolio's line relates to is physical attraction. Physical attraction is a very dominant theme in the play, which is evident when we see just how much Romeo's feelings of love are governed by physical attraction; he even equates love with physical attraction, which is evident in several places throughout the play.
this will never be the same