The real story behind the hatred between the Capulets and the Montagues is never explained. However, readers can speculate. It is clear that the hatred is strong and the feud has lasted a very long time.
The two families, the Capulets and the Montagues, are very wealthy and powerful. Undoubtedly, jealousy and rivalry may be important factors in the feud. The two families both want to be the most powerful in Verona.
It is entirely possible that the feud started because of an act of betrayal by one family against the other. For example, let us assume that one family had offered a son in marriage to the other in order to cement their relationship. Had the other family initially promised to accept and then married the daughter to the son of another powerful family instead, it would have caused very bitter feelings.
Another possibility is that both families started nasty rumours about the other in order to cause them embarrassment. Although rumours are often very far from the truth, it is a sad fact that they often spread very quickly.
Here is a plausible story that may explain the real reason for the hatred. The Montagues of long ago were planning to have a big celebration in honour of their esteemed family heritage. Mrs. Montague, a friend of Mrs. Capulet, decided to share the details of their celebration with her friend. She revealed the dress that she would wear, the entertainment that would take place, the food and drinks that would be served and the prizes that would be given away. Although she appeared to be excited to learn the details of the celebration, secretly Mrs. Capulet was very jealous. She was not content to be less important and less popular than Mrs. Montague. She suddenly had an idea. It was time to celebrate her family's proud Verona heritage and her son's 15th birthday which would take place next week. It was a wonderful excuse for a party.
To dampen the Montagues' celebration, Mrs. Capulet quickly dispatched a messenger to announce to all the prominent families of Verona that there would be a celebration at the Capulets' home that weekend. The messenger announced that it would be a party unlike any Verona had ever seen before. When she heard the announcement, Mrs. Montague was so incensed that she almost decided not to attend, but she did. However, the day of the celebration, to her horror she noticed that Mrs. Capulet's dress was similar to the one she had described and the food, drinks and entertainment were similar to what she had planned. She could not believe her eyes! She later made it clear to Mrs. Capulet that she and her family were no longer welcome to attend the Capulet's celebration the following week. Thus the feud was born.
Of course the preceding scenario is a fictional one, but it may explain the real story behind the hatred. In any case, Shakespeare was depicting human nature. In many cases, people no longer remember the reasons for particular feuds and they may believe stories and rumours that are not necessarily true. With the feud, Shakespeare demonstrates that for many people it is difficult to forgive and put feuds behind them. In the case of Romeo and Juliet, it took the deaths of the young couple for the families to come to their senses and finally end their long feud.