It’s a reaction to the ending scene in the play. Jim, while wanting to show Laura how unique and wonderful she is, hurts her when he reveals he is engaged. However, Laura, in many ways may have still been comforted, reassured, and newly confident still, as her self-belief may have changed as a result of his perception of her strength and uniqueness, like that of the unicorn. All Amanda could see was the devastation of the lost opportunity that she thought was there for her daughter. Amanda's over-reaction uproots the stability that Tom provided in the home, by forcing him to leave. Tom provided both financial stability, and was a true emotional support for Laura. Laura is left with her mother, who has just finished stating how helpless she feels her daughter to be. Laura's hidden smile may reveal to the reader her growing sense that she is capable, and that she now knows there are people who believe in her (Jim and Tom). It may also be a smile that reveals to the reader Laura's understanding that Amanda needs to feel like Laura cannot do anything herself because it gives purpose to Amanda's life. If Jim married Laura, or Laura got a job, Amanda would have nothing to complain about, to nag about, to fuss about; she would be miserable. Laura in a way, knows this, and Laura also knows that she may spread her wings someday and fly away from her mother. It may be a smile of sympathy, understanding, hope, all of the above, but certainly not a smile her mother should see.
Laura because of her physical handicap and nervous and timid nature lives in constant dread of her mother Amanda. In Sc.VI Amanda announces to Laura that Tom is bringing home one Jim O'Connor. Laura remarks that she was acquainted with a Jim at high school and that "if that is the same one that Tom is bringing to dinner-you'll have to excuse me, I won't come to the table." Soon Jim arrives and with great difficulty Amanda forces Laura to interact with Tom iin ScVII. In Sc.VII Tom cures Laura of her inferiority complex, but the dramtic irony is that Amanda is not aware of it.
Amanda meanwhile is hopeful that Jim will propose marriage to Laura but their hopes are shattered when Jim reveals to Laura that he is already engaged to another girl called Betty. For a moment it looks as though Laura is going to have another one of her nervous breakdowns, but heeding Tom's advice,"think of yourself as superior in some way" she is successfully cured of her inferiority complex. Amanda even at the close of the play is not aware of the momentous change that has taken place in her daughter's life and assuming that she must be emotionally upset comforts and consoles her. But Laura is no longer downcast but she faces her mother boldly and smiles confidently at her indicating to her that she can handle this disappointment and all future disappointments in her life confidently all by herself.