Does Romeo's love of Rosaline in the beginning of the play weaken the love he claims to feel for Juliet at the end of Act 1?
On the contrary, it seems as if Romeo's unrequited desire for Rosaline and her aloof and cold response to him strengthens his love for Juliet.
Romeo had previously confided in Benvolio, telling him about his concern about Rosaline's frigid attitude to his advances, saying:
"... she'll not be hit
With Cupid's arrow; she hath Dian's wit;
And, in strong proof of chastity well arm'd,
From love's weak childish bow she lives unharm'd.
She will not stay the siege of loving terms,
Nor bide the encounter of assailing eyes,
Nor ope her lap to saint-seducing gold:
O, she is rich in beauty, only poor,
That when she dies with beauty dies her store."
He is quite distressed by Rosaline's rejection and claims that she has made a vow of chastity and is therefore not open to his persuasions or his charms. Rosaline generally ignores him. She refuses to listen to him or to return his loving glances.
From his remarks, it is clear that Romeo is completely infatuated with Rosaline. She is beautiful and intelligent and Romeo believes that there is no other girl who can match her beauty. Benvolio informs him that he had no one else to compare Rosaline to. He suggests that he and Romeo should attend the Capulet's feast that night, at which Rosaline and other fair maidens will be present, so that Romeo can make a more accurate and discerning comparison.
When they attend the Capulet's feast in disguise, Romeo sees Juliet for the first time and is instantly attracted to her. He says:
"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."
When the two of them finally meet, Romeo is completely smitten by Juliet for, unlike Rosaline, she spiritedly indulges him in conversation, unafraid and open to his suggestions. The wordplay between them spurs him on. When he eventually leaves, Romeo states that he will never experience such happiness as he had experienced that night (by meeting Juliet).
Rosaline has been forgotten.