In Romeo and Juliet, what would be a good quote from Act 1 that shows Romeo's lack of commitment to Rosaline and why?
I think that a good quote to show Romeo's lack of commitment to Rosaline would be his emphatic description of Juliet upon first seeing her. Prior to this moment, Romeo had been crushed because Rosaline had abandoned love, and thus rejected Romeo. Consider the language he uses to describe his condition: "I am too sore enpierced with his shaft/ To soar with his light feathers, and so bound," and "Under love's heavy burden do I sink" as well as "Out of her favour, where I am in love." These descriptions convey how Romeo might have been more in love with the idea of being in love than a conviction towards Rosaline herself. Romeo does not express anything that shows a feeling of love towards Rosaline, specifically. It is more of an expression of what he considers to be love. Rosaline is not named as a part of it. There is not much to show that Rosaline has specifically done anything to imprint herself upon Romeo's heart.
The moment Romeo see Juliet, thoughts of Rosaline completely disappear. Consider the language used when he sees her:
O, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiope's ear;
Beauty too rich for use, for earth too dear!
So shows a snowy dove trooping with crows,
As yonder lady o'er her fellows shows.
The measure done, I'll watch her place of stand,
And, touching hers, make blessed my rude hand.
Did my heart love till now? forswear it, sight!
For I ne'er saw true beauty till this night.
There is nothing about Rosaline which seems to exist any longer. Romeo's passion towards Juliet reflects a lack of commitment to Rosaline. Romeo's concluding idea that he "ne'er saw true beauty till this night" is a clear indication of a lack of commitment towards Rosaline. He is able to abandon her rather quickly when confronted with the presence of Juliet. It is Shakespeare's genius to show how "love" can be interpreted with so much passion, but also be so very fickle in the process.