One must define a few different things prior to writing a thesis. First, one must define what the focus of the essay will be (essentially, what will the point of the essay be?). Second, one must define what mode of rhetoric (type of essay) will best suit the purpose, or point, of the essay. For example, is the essayist comparing and contrasting, stating a cause and effect, defining, or persuading? The defining of the type of essay will help the essayist define what type of thesis will best suit the purpose and type of essay.
In order to construct a thesis for Alice Munro's "How I Met My Husband," one must first define what it is he or she wishes to focus upon. For one, the essayist could focus upon the numerous themes highlighted in the text. By doing so, one could define the success for which each theme is defined (resulting in a cause and effect essay). One could also examine, through a causal chain, the chronological order of events which lead the main character through the text. A final example of an essay's focus would be to persuade the reader that Edie's naive character leads to her own enlightenment.
A thesis, then, must define the point of the essay and what the essayist intends to prove. For example, if the essayist intends to argue that one decision can change the entire direction a person's life could take, he or she could define a thesis as stating that Edie's choice to dress up and put on makeup pushes her life in a very unexpected direction (illustrating a cause and effect). This simplistic choice marks a change in life greater than simply putting on makeup and nicer clothing.
Keep in mind that a thesis does three very specific things: 1) it presents the topic; 2) states an opinion (either an assumed or stated opinion); and 3) offers a "blueprint" for the layout of the essay (defining how the essay will progress). Here is an example of a strong thesis (which does not relate to Munro's story).
Alexander Pope proves that a poet who writes literally using figurative language is better than one who pretends to write and wastes time “practicing” things which mean nothing.
Here, the topic is Pope and figurative language; the opinion is that literal writing (including figurative language) is better than than wasting time practicing the art of figurative language, and that the "blueprint" will look at both literal use (which is successful) and wasted practice (which is unsuccessful).