In literary analysis, a thesis is usually informed by themes that the writer has identified throughout a piece or pieces of literature. In your class this semester, your professor has identified works that have common themes of love and desire. All of the works you're reading address love and desire in some way, sometimes very obviously and sometimes more obliquely.
You may identify other themes in these works, especially related to your primary field of study. For example, if you study psychology, you may recognize themes related to the mind, cognition, or behavior related to love and desire. If you study economics, you may recognize themes of class, work, or exchange and trade in relation to love and desire. Literature can be very open to interpretation, which is one of the things that makes it so fun to study.
When you read a text, think about what you see in the story beyond the plot.
These are the things that help you identify the themes and what you think the author is saying about those themes.
Once you've identified some repeating motifs in the story, start asking yourself "why" and "how." Here are some examples that aren't specific to any story, but show you how to begin to probe the details you noticed when reading.
Once you've identified some recurring imagery, language, and themes, and begun to question the role of these things in the book, you can start to form your thesis.
Your thesis statement is your claim about what the novel is doing and how. This statement is what you think the novel is about beyond the plot summary.
Some examples of thesis statements include:
"Arna Bontemp's novel Black Thunder represents and reflects on shifts in consciousness happening in the African American community during the Harlem Renaissance."
"The distinct treatments of space and belonging in novels by Waguih Ghali and Leila Sebbar reveal the multiplicity of ways in which nationality can be understood."
"Countee Cullen’s poem Bright Bindings' is about expectations. But it is not simply about disillusionment and disappointment. The form of the poem, and the words he uses, show that even when something or someone fails to live up to what we expect, that failure is in some ways anticipated."
Your thesis statements tells the reader what you think the author was trying to say in writing the piece.