What's the biggest mistake beginning writers make?

Dear Literary Ladies,
What is the biggest mistake or miscalculation aspiring writers make when first start sending their work out? There's usually no feedback, so what's the single most important lesson to keep in mind?

It was during the six years when I was editor of McClure's magazine that I came to have a definite idea about writing. In reading manuscripts submitted to me, I found that 95 per cent of them were written for the sake of the writer never for the sake of the material. The writer wanted to express [her] clever ideas, [her] wit, [her] observations. Almost never did I find a manuscript that was written because a writer loved [her] subject so much [she] had to write about it.

—Willa Cather, from an interview, Omaha World-Herald, 27 November 1921