In Shakespeare's Romeo and Juliet, what are reasons why Romeo is affected by the conflict between the Montagues and Capulets?
There certainly are several reasons why Romeo in Shakespeare's Romeo and Juliet is emotionally affected by the feud between the Montagues and Capulets.
One reason is because he simply finds the feud to be distressing. He very clearly understands all the underlying reasons behind the feud. One reason is because the two families hate each other, but the other reason is because the two families love too much, more specifically, they love power too much, which creates jealousy. We see Romeo explain his understanding that the feud equally has to do with both love and hatred in the opening scene:
Here's much to do with hate, but more with love.
Why then, O brawling love! O loving hate!
O any thing, of nothing first create! (173-75)
Other ways in which the feud emotionally affects Romeo has to do with his love life. Both Rosaline and Juliet are descendents of the Capulet family. Though Rosaline does not reject Romeo simply because he's a Motague, it's evident that the feud does separate Romeo from Rosaline, as can be seen in the fact that Benvolio and Mercutio persuade Romeo to crash the Capulet ball with them in order to give Romeo a chance to see Rosaline. And, of course, the feud separates Romeo from Juliet, whom he must marry in secret.