Be happy! You mustn’t be sad (= don’t be sad). (Essential Grammar in Use)
I’m confused whether it’s possible to order other people to have certain emotional accomplishment. Do the two sentences really mean that, or do they just demand the hearer’s outer expression showing pleasure, not showing unhappiness?
The imperative mood is not only used for commands or requests, but also for exhortations. Be happy is an exhortation, not a command.
Don’t worry, be happy.
Source : Link , Question Author : Listenever , Answer Author : apaderno