When Romeo arrived without knowing the mesage of the correspondent messanger for explaining the crazy plan of Juliet about faking her death, he saw Juliet's body and kill himself right next to her believing that Juliet is really dead. When Juliet awaken. She saw the dead body of Romeo and so she stab herself using a dagger, for real she killed herself. Montagues and Capulets knew it and blame themselves then decided to end the feud between the both of them.
