Therese is a character who, from the very first introduction the reader is given to her, it is clear feels she is destined for more than her current monotonous existence married to the invalid, Camille, and trapped working in a shop with her mother-in-law who dotes upon her son. Note the following information that the reader is given about Therese and her upbringing in Chapter Two:
The cloistered life she led, the debilitating regimen to which she found herself subjected, failed to weaken her thin, robust form. Only her face took a pale, and even a slightly yellowish tint, making her look almost ugly in the shade. Ever and anon she went to the window, and contemplated the opposite houses on which the sun threw sheets of gold.
Naturalism was a movement that argued so much of how humans develop was a result of environment and the conditions in which they were raised. It involved the incorporation of the ideas of Darwin and in particular his Theory of Evolution with realism. Therese, in this quote, is shown to grow up in squalid conditions where she is forced to lead an unfulfilling life thanks to Camille's health. However, note the way that she is attracted by the irresistible allure of wealth that is represented in the "opposite houses" that, thanks to the sun, appear covered in "sheets of gold." This metaphor is no accident: it represents everything that Therese wants and feels she deserves. Her character is defined by dissatisfaction with her upbringing and her surroundings, and therefore this prepares the reader for her willingness to do anything to improve her lot.