In A Streetcar Named Desire, what causes Blanche's distortion of reality?

Blanche's
distortion of reality is a coping mechanism she uses to survive, and at the heart of her having
to use a coping mechanism is desire (the title of the play, of course) and the discrepancy
between how Southern women are expected to act, and how they do and must act. 

When she distorts reality by insisting that her former male friend is going to come and
rescue her, she is attempting to cope with her situation and to hold on to her dignity.  As a
woman in a patriarchal society, she is forced to rely on men for all of the things a person
needs to survive.  Blanche cannot be herself.  Stanley can fulfill his desires, and so can
Stella.  Blanche cannot.  Society won't allow it.  When Mitch discovers that she is not the
ideal, virginal-like maiden he thought she was and rejects her, and when Stanley rapes her and
is in the process of getting away with it, Blanche attempts to hold on to her dignity by
insisting at least one man still thinks her worthy of attention.  Of course, her attempts are
futile.  Stella believes Stanley and takes his side, and Stanley wins the battle for Stella that
he an Blanche have been fighting.  Blanche is hauled away. 

Blanche does what
she can to survive and to hold on to her sanity and her dignity.