We also say: "You are quiet today. Everything OK?" Taking the section, 'You ARE quiet': 'quiet' as a noun means: (a) silence, calm :"The early morning quiet" (b) free of disturbance or interruption BY OTHERS So, with 'quiet' as a noun, your sentence would mean: “You should be silence.” Do you think that a metaphor is the intended meaning, rather telling someone not to disturb others by talking or making noise? Take another example: ‘happy’ is the adjective, and ‘happiness’ is the noun. “They are happy to see me.” “You should be happy that you still have your health.”