Why doesn't Romeo know Juliet isn't really dead in act 5 of Romeo and Juliet

The letter Juliet told Friar John to send go Romeo did not get to him in time, so when he heard the news about Juilet, he thought she was dead.

taking into account that a theme is the central topic a text treats and can be developed through a story, play, poem etc. the following are themes that can be further developed:  

my best friend jeffrey has many unusual interests. (the story could be about each of the unususal interests)

once a man gives his soul over to evil, he can never go back. (this could be a saying or a proverb)

the three sheepdogs lay around the yard all day. (it could be developed detailing how each of these sheepdogs lay around)

a priest's collar has an unusual effect upon people. (it could be developed detailing the different effect upon people)

innocence is a powerful force in a world full of corruption. (it could be developed detailing how is a world full of corruption)

king lear is a play shakespeare wrote later. (it is just a sentence and doesnt give any idea or topic to be discussed in details.

evil often wears the disguise of innocence. (this could be a saying or a proverb)

samuel gompers was an extremely important person in the american labor movement, being him an employee all his life he devoted himself to fight for the rights of his pairs, he believed they all had the right to have better thing and that they all were working for them but the factory workers were greedy, ambitions and socialist people for him who he didn't have a good concept of.

he gave so many speeches and led so many strikes and protest to fight for equality which he considered employers didn't want to accept, so he had an absolutely negative opinion of them.