Themes are ideas that run all the way through a literary text. They are great to analyse because you can discuss the writer's intentions: what have they done? Why? What are they trying to make the reader and audience think and feel? This allows your analysis to be extended and developed, allowing for you to write comprehensively about a given literary text.
In Romeo and Juliet there are many themes such as:
All three themes interlink, as Shakespeare wanted to illustrate how love, conflict and family intertwine with one another. He also wanted to show that sometimes, through love, we can also hate.