Anybody here know how to write a good thesis statement?

I'm writing a paper on the story "The Diamond Necklace" using Marxist criticism, and I'm trying to write a thesis statement on the story. Could you please look at this paragraph and think of a thesis statement? I've been working on it all day and I can't think of one.

My opening paragraph needs a thesis statement:

"Focusing on class distinctions, the elitism of the upper class, the struggle of the lower class, Mathilde wants to become a member of the upper class so much that she allows obsession to drive her to make foolish decisions.

This is the story I'm write a paper about:

http://www.classicreader.com/book/518/1/

Thanks.