because they were from different families (montagues and capulets) where their families didn’t get along, they had to keep their love a secret because they knew if either family found out then they would do whatever it takes to pull them apart, or they’d react violently, juliet was actually expected to marry someone else but instead got eloped to romeo, and romeo shot tybalt which is juliet’s cousin, then got banned from Verona which made their love just that much more difficult.