Here's one thing a thesis shouldn't be: an observation. Unfortunately, you can't just point out something true about a novel or short story, and describe that something for five paragraphs. Or you can, but only if you want to see "OK...annnnnnd?" or "So what?" scribbled in the margins of your paper.
Coming up with an observation, as this Sparkler has, is a good first step:
In both the novel Life of Pi written by Yann Martel and the movie The Black Stallion by Carrol Ballard, the protagonists start their lives as innocent and naive boys but grow up mentally with the help of experience, heart built relationships and actions that show they are different from most boys.
Both protagonists mature. Got it. But that's just a good observation. It's not a thesis because it doesn't express an opinion. It doesn't propose an argument that the writer will flesh out in his or her supporting paragraphs. It will make readers think, "Yeah. Obviously."
If you realize that you've got an observation on your hands, rather than a thesis, push yourself farther. The goal is to come up with an argument that's defensible, but that reasonable people could disagree with. For example:
--In Life of Pi and 'The Black Stallion," adulthood is linked with familial cruelty: the protagonists gain maturity only by rejecting their mothers.
--Martel and Ballard make a compelling case for the benefits of physical peril, suggesting that it's character-forming for children.
You want to make readers think, "Wait...really? I'm not sure I agree with that," or, "Huh. That's interesting. I think I agree." Then, in your supporting paragraphs, you vigorously argue the merits of your position, convincing your readers that your opinion is well reasoned.
Contact me at firstname.lastname@example.org.