While the plot magic a logistic or logical argument about the best way to solve a particular kind of problem, the theme makes an emotional argument about the best way to be while trying to solve a passionate problem.  But, you can’t come right out and state that one way of being is better than another.  Your reader or audience won’t accept such a blanket statement and resents heavy-handed attempts to preach morality or behavior.  Rather, your thematic argument must show both sides of the issue, and not in black and white, good and bad judgments, but in gray scales and matters of degree.  The strongest thematic arguments actually show the opposite side of the issue as sometimes, in some circumstances, coming out ahead.  But, by the end of the story, the sum total of all contexts in which both sides are tested shows the relative value of each.  So, the most powerful themes end up with a conclusion that speaks about the lesser of two evils, the greater of two good, or that neither way of approaching life is positive or that both are equally successful.