The absence of an explicit subject allows these questions to be asked in a variety of contexts.
Some of those sentences would mean the following if you are directly addressing someone:
Why are you pretending?
Why are you being so serious?
Why don't you go out and make a scene?
Or they could be actual questions about a third person:
Why is she|he pretending? What would cause a person to pretend in that situation?
Why is he|she being so serious. What's going on that he|she would be so serious now? It seems strange for him|her|them to be so serious.
Others could be rhetorical and refer to oneself indirectly as "one":
Why should anyone pretend? Why should I pretend?
Who should one be? Who should I be?
Why should anyone try? Why should I try?
And the most famous one of all:
To be, or not to be?
The verb BE is often omitted. Here the subject is provided by context.