The simple answer, if there ever is one with Shakespearean work, is that they both die as a result of social strife. Had there not been such an intense hatred between the Capulets and the Montagues, there would not have needed to be an elaborate plan where one fakes their own death and the other fails to recognize the intricacy. Juliet's plan to take a potion that would enable her to look dead so that Romeo would be able to recognize the ruse and then both of them could escape together is something that only comes about because of the weight of social expectations providing inertia to their own desires to be with one another. Through bad circumstance (or fate), Romeo does not understand the plan and misreads Juliet as actually being dead. Believing this to be the case, he takes his own life. When Juliet awakens from her slumber, she sees Romeo dead and takes the dagger to end her own life. The bodies of the two lovers is a testament to how they could not live in the world of Verona, and how their love was transcendent beyond it.