Imagine you'd told a bunch of students that their papers must be structured and controlled by a one-sentence statement called a "thesis."
Could they be forgiven for thinking that they are being asked to make up some trite garbage before they've even read the text carefully and hang onto it for dear life whether or not it is borne out by evidence? I think they could.
The thesis is your argument. The thesis is the thing you stand behind. Everything in the paper is supposed to lead back to the thesis. We tell students that writing is a process, but the concept of a "thesis," in my experience, encourages intellectually untenable linear, top-down writing strategies and discourages revision.
I tell my students to come up with a hypothesis, not a thesis. A hypothesis is what you think is going on, what you think you're arguing, for now. Then you look at passages that you think are relevant. You analyze them, unbiased. You're checking your hypothesis, not desperately cherry-picking support for it. And if your hypothesis doesn't receive much support from the text, then you change it.
This is completely counterintuitive to a lot of students. Change your thesis?
Yes, because it's not a thesis yet; it's a hypothesis. The point of an essay is not, actually, to defend an argument no matter what the argument is. It's to develop an argument worth defending. That's harder than coming up with a "thesis."