"Pyramus and Thisbe," the play-within-a-play, is based on the Ovid myth (found in Ch 4 of "Metmorphoses.")  Shakespeare's audience, at least a good number of them, would have been very familiar with this popular sentimental myth and hoping for a better outcome for these lovers than the ones the play presents, or at least had a good laugh at this familiar romantic tale told by such an unseeming company of players. 

(For an idea of how this might have looked, think of a modern day equivalent of a movie/novel, played in earnestness by unlikely actors, say Paris Hilton in a Princess Di bio-pic!)