Why the morality play Everyman the play begin with messenger and end with doctor ?

These guys are like bookends to a frame story. The Messenger has no role within the story of the play itself, but simply speaks the prologue outlining what the play will be like. The doctor is as well a generic character who only appears to speak the epilogue at the very end of the play. His equivalent in the Dutch play Elckerlijc is simply called 'Epilogue'.