I believe that it is Romeo and Juliet's PERCEPTION that their families would not accept their union that led them to keep everything secret. If you remember, at the Capulet party, Tybalt recognizes Romeo's voice and tells his uncle that a Montague has crashed the party. Mr. Capulet's response was that Tybalt should not worry about the intruder and that he had, in fact, heard that Romeo was a fine young man. After the Prince had made his proclamation that the next one to disturb the streets with the feud would be put to death, Mr. Capulet had even remarked to Paris, "'tis not hard, I think,/
For men so old as we to keep the peace", hinting that he would not be as willing to engage in the fighting any lionger. It was TYBALT who went after Romeo the next day to defend the family's "honor" at having their party crashed by a Montague. Romeo had not interest in fighting him, but Mercutio decided to goad Tyblat on. With the slaying of Mercutio, Romeo reacted out of anger, thus feuling the feud with Tybalt's death.
So as to your question, it was the youngsters' perception that the other would not be accepted (based on past performances in the streets of Verona) that led them to keep silent. If only Juliet had overheard her father remarking about Romeo being a fine young man, the story might have gone from a tragedy to a fairy tale.
Because their families were enemies. Their love would never have been approved. Juliet, most likely, would have been shipped off somewhere to live with a relative, to school, or marriage far away from Romeo.
Romeo's fate may have been similar. It was not uncommon for young men to be shipped off for studies abroad.
At any rate, the families would have done all within their power (they had a lot of it as they were both affluent in the community) to keep the two separated.
Romeo and Juliet's families have beed feding for so long that even their fathers do not know what it is all about. The only thing that is known to the Capulet and Montague families is that they are to hate one another; as ridiculous as this may seem. Even the servants of the Capulet family hate the servants of the Montague family. There is no reason; no event that can be recalled by anyone in Verona.It is bigotry in its purest form. Each family is intolerantly devoted to their own prejudices.The result of this unresonable hatred is neither Romeo nor Juliet can confess their love for one another.
Another reason they cannot confess their love to their families is, they are young adolesents. What teenager wants to tell their parents something they know will get them in trouble. Yes, it was love at first sight. they were irresistably drawn to one another. However, let us not forget that Juliet did confess her love to the Nurse, and Romeo confessed the same to Friar Laurence. These adults to whom they felt a deep connect were shocked by the news. And though they help them carry out the marriage, each told their charge to forget about one another. In the end Romeo and Juliet were betrayed by the adults to whom each confided. They knew their families would have beated, banished, or even killed them for crossing the line of the families' long-standing feud.
Romeo and Juliet couldn't tell their families because their families hated each other. Rome (the Montagues), and Juliet (the Capulets) had been fighting with each other for quite a long time. The killing of Mercutio, and then Tybalt then escalates this feud. By the end it is obvious that neither of their families would support the marriage, so therefore they could not be wed.