In Romeo and Juliet, the Montagues and Capulets are essentially “at war” with each other. Almost anytime their paths cross, swords are drawn, and violence ensues. Most reasonable people would agree this level of bitterness is stupid, because it serves no purpose and only causes problems.
By refusing to reveal why the two families have such a bitter feud, Shakespeare ensures that the readers’ personal biases do not interfere in interpreting the message that hating entire groups of people solely over their family memberships is stupid.
Beyond the scope of the story, any reasonable person would agree to the principle that hating an entire group of people without knowing the content of their individual characters is silly at best. This can relate to race, religion, gender, profession, family, and so on. However, people’s personal biases get in the way of this all the time.
For example, would you dislike someone solely for their career? Almost everyone would answer that with no.
Do you dislike lawyers? Deep down, people genuinely still believe in the aforementioned principle of looking at individual character, but emotional biases can make it easy to forget that principle. Someone who has had bad experiences with greedy dishonest lawyers might answer this question with “I hate lawyers.”
Back to Romeo and Juliet, let's assume the family conflict had started 50 years earlier, when Capulet killed Montague’s mother in response to Montague's attempt to kidnap Capulet's sister? In this event, it might make sense for Montague and Capulet individually not to forgive each other, but for all family members to hate everyone on the other side is still unfair. However, if Shakespeare had revealed this to be the cause of conflict, I think many readers would have rationalized it, and forgotten that most Capulets and most Montagues had nothing to do with that unfortunate event.
Readers invoke their own biases all the time, maybe because of personal experiences or maybe because of gut emotional reactions. Shakespeare does not want that to happen with the Montague/Capulet conflict, and so he consciously decides not to mention the reason for the feud.
In effect, the reader’s default perspective is that the conflict is stupid. Now, the reader can more easily understand Shakespeare's message that it is stupid in any circumstance for entire groups of people to hate each other over something as arbitrary as family membership.
When Juliet says: “What's Montague? It is nor hand, nor foot,/Nor arm, nor face, nor any other part/Belonging to a man. O, be some other name!/What's in a name? That which we call a rose/By any other name would smell as sweet” (Act II Scene II), I think this is precisely the point she is trying to make.
Source: reddit post