Shakespeare uses the mechanicals to show how ambition can distort reality, making one believe in an illusion of reality rather than what is truly real. The mechanicals are a group of menial laborers from Athens. They are uneducated and have never performed before in their lives; yet, they feel a driving desire to perform a play before Duke Theseus that they have written themselves in honor of his wedding day. Their ambition makes them visualize performing a grand play, when the reality is that they simply do not have the knowledge and skills to perform at the level they are envisioning. The reason why they are so disappointed when it looks like they will not be able to perform their play is that, due to their ambition, they all have visions of grandeur.
The mechanicals begin to doubt that they will be able to perform the play when they cannot find Bottom, whom they see as the star of their show, the only one capable of making their performance a grand one. Quince determines that if Bottom does not show up, it will be impossible for them to perform the play, for he believes that Bottom is the only man in Athens who can play the part of Pyramus because he is the best looking and has the best voice, as we see in Quince's line, "Yeah, and the best person too; and he is the very paramour for a sweet voice" (IV.ii.11-12). By "paramour," Quince means to say "paragon," meaning model. The mechanicals' view that Bottom is their star is actually influenced by Bottom's own conceited and unjustified opinion of himself, as we see him declare, "If I do it, let the audience look to their eyes; I will move storms" (I.ii.22-23). Hence, we see that the mechanicals' vision of grandeur is influenced by Bottom's own vision. Therefore, the mechanicals are disappointing when they think they will not be able to perform the play because their ambition has lead them to believe that they can give a splendid performance, but not without Bottom.
The actors are disappointed because they know that if the Duke liked their play, he would reward them with so much that they would be happy forever. Now, that Bottom is not going to be there, they can't perform, and therefore, they will not get anything from the duke.