Because Kurt Vonnegut is a well-known and experienced writer, (therefore) you should trust his advice and apply it to your work, since he is a published author.
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The first rule of Kurt Vonnegut's How to Write with Style is for the writer to find a subject that they care about. From personal experience, when I write about a topic that I care about, I am invested in the work, ideas flow to me freely and I want it to turn out as best as it can, whereas when I am writing about a topic that I am indifferent about, I tend to have a harder time developing arguments and end up just writing to please the teacher. When someone has a strong feeling towards a topic, they tend write with ease and want their opinion to come across clearly and prove to the reader why their opinion matters. However, when one writes because they are given a prompt and have no other choice, they tend to spend more time brainstorming about ideas and then feel rushed to finish their work. Altogether, when writers care about a topic, they are more likely to write with ease and state their arguments more clearly, producing a more well-written paper.