People should not base love on someones family or place.
Act 1 Scene 5
"' Then have my lips the sin that they took.' 'Sin from my lips? O trespass sweetly urged; Give memy sin again. ( Shakespeare pg 1016)
This relates to the theme Even though Romeo and Juliet are from rival families in Verona. They look pass this because their love is able to look over the one flaw.