Why do Juliet's parents try to arrange her marriage to Paris?
In part, the answer is because arranged marriages between the elite classes were the norm. In arranged marriages, the thinking was to create political and social allegiances. Lands and monies could be drawn together between amicable families, thus increasing the power of both families. Love marriages, at the time of Shakespeare's writing, were just coming into vogue.
Shakespeare appears to be considering the consequences of the new movement and the possible re-alignment of patriarchy, but does so by safely removing the conflict to another part of the world. “Let’s see what happens when the old norms are removed, when people in authority do not follow the proper rules, and when young people subvert the system,” Shakespeare seems to be saying. Looked at in this light, the play is less a romantic plea for choice than a warning to the populace about the dangers of upsetting the system and the problems caused by players who do not follow the established rules.
However, we do see the influence of the shift toward love marriages. Lord Capulet tells Paris, "Her will to my consent is but a part, / And she agreed, with in the scope of her choice" (1.2.16-17). Here we see some capitulation to love marriages, but a very limited choice. Juliet is free to choose, ostensibly, so long as her selection meets with her father's approval.