Voice in writing allows readers to feel like they connect to a real person within the words. Voice marks the difference between textbook and dictionary writing and the kind of writing you hug in your arms because you finally feel understood and bonded to the writer. It's the kind of writing that you can't resist. You have fallen in love.
Voice emerges from sentence variations in two ways: the patterns of sentence construction and also length (as in the number of words in each sentence). It's just like in speech. In speaking, if every sentence began with a subject and verb and ended with the object, you would sound like a robot. Monotonous. Agonizingly boring.
So you let the subject and verb appear in different places. Use some introductory clauses, some glorious prepositional phrases, or some enthralling adjectives. By doing so, you create rhythm. As you write, don't forget to add in the trick of the very short sentence. It's simple. And within your varied sentence lengths or openings you'll use cool punctuation marks; using advanced grammar like the semicolon creates a voice as well. Finally, the best creators of voice know the power of two things: the colon and the dashes. Try writing down sentences--amplifying with dashes what you really want the reader to know--and see what happens. Voice means we're hearing you in the paragraph. Use these variations, and voice seeps out onto the page (and remember that parentheses work like whisper, a little secret between you and the reader).
We'll hear you, and we won't be able to resist you.