I would argue that the most important theme in Romeo and Juliet is that of fate. This is touched on in the Prologue to the play, which refers to the two as "star-cross'd lovers." From the moment they fall in love at the ball (which itself wouldn't have happened had Capulet not restrained Tybalt from fighting with Romeo) events seem to conspire against them. Tybalt and Mercutio clash in the streets, leading to the former's death at the hands of Romeo; Juliet is engaged to be married to Paris; plague prevents Friar John from reaching Romeo to inform him of Friar Lawrence's plot. In short, everything that could go wrong does, and the two lovers perish as a result. One might as plausibly argue that the theme of hatred or conflict, and love's ability to transcend it, is equally important, but I would argue that the role of fate, which Shakespeare examines in other plays as well, is fundamental to this tragedy.
i think basically, love. although you could argue tradegy