How does the fact that Romeo is a Petrarchan lover weaken his and Juliet's love?
I want to be clear on this first point. I don't think that it weakens Juliet's love for Romeo. I buy into her love for him, despite the fact that she is 13 and fell in love at first sight. While that's trite, and she is very young, I can believe it.
Romeo's love for Juliet, however, I've never fully believed. A big part of that is because Shakespeare introduces Romeo as a Petrarchan lover. A Petrarchan lover is someone who falls in love easily with a girl/woman who is unattainable in some way. That causes the lover to be so love sick that all he does is mope about and complain. That's exactly how Romeo is introduced in Act 1.
He complains to Benvolio about how depressed and sick and sad he is, because the beautiful Rosaline doesn't reciprocate his love. Benvolio does everything imaginable to cheer up Romeo, but Romeo is unaffected. Because of that opening introduction to Romeo, I feel that the case for his love of Juliet is weakened. He sees Juliet, and within 14 lines of dialogue, he is kissing her. Keep in mind that 12 hours prior, Romeo was inconsolable over his love for Rosaline. I just don't buy Romeo's initial "love" for Juliet.