Probably depends on who you ask. Obviously, as the previous post points out, much of it is about love. It is about the crazy things people do when they think they are in love and about how the love of these two tragic kids can bring the feuding families back together.
One of the major themes is also tragedy, the deaths in the various families brought on by stupidity and bad chance or "fate" and the way that it tears people apart before it brings them together.
Another theme is fate, the idea or question of whether or not it decides peoples lives or if people decide their lives themselves.
The major theme in Romeo and Juliet is love. There is the idea of love conquering all, love being the downfall of man, and love developing the individual. In the play, it is through the love shared between Romeo and Juliet and the ultimate sacrifice that the two make that causes the feuding families to realize that love and forgiveness should overcome years of hate. It is through this love story that Shakespeare expresses the importance of the individual. It was customary for children to become engaged at a young age without any say in who to marry. Many of these marriages resulted in love loss which was portrayed through the Capulet relationship. The theme of love also addresses different types of love. The first is the sensual type found between Romeo and Juliet but there is also a second type which is parental love. This type of love is found between Romeo and Friar Lawrence and Juliet and the Nurse. Friar Lawrence and the Nurse are surrogate parents to Romeo and Juliet due to the absence of parental love from their biological parents. This is another point that Shakespeare makes throughout the play. Take note of the scene where Lady Montague talks about Romeo’s depression, but instead of going and talking to her son about it she sends Benvolio.