I have just finished arguing with a would-be writer who a) is convinced that passive voice is evil and must be avoided at all times, and b) has, it turns out, no idea at all what passive voice actually is.
I am therefore going to rant.
Passive voice is not when something has been allowed to happen, nor is it a literary device, nor is it any sentence containing “was” or “were. Passive voice is a grammatical term, not a literary device. Passive voice is a specific type of sentence structure, in which the subject of the sentence is acted upon by someone or something else, rather than acting: “The time was given to her by me,” is passive voice; so is “She was given the time by me.” But “I was giving her the time” is active voice (past progressive tense, to be precise), and “Giving her the time,” is a gerund phrase.
And the active voice is not ALWAYS preferable. It depends on the emphasis one wants in the sentence. If you are writing a simple descriptive paragraph, then the simple active-voice sentence: “Her dog had saved her life” is probably what you want. If, however, you are writing a paragraph in which the protagonist is considering the purpose of her life and musing about her past, then “Her life had been saved by her dog” may be exactly what you want, because in context, “Her life” is the important thing, the thing she is thinking about, the thing that requires emphasis, because that (not the dog or the event) is the focus of the paragraph.
Passive voice can be absolutely necessary in a sub-clause. For instance: “The ambassador, having been insulted, returned home and persuaded his country to declare war.” That clause, “having been insulted,” is passive voice; you can tell because you can add in the “by whom” and the phrase still makes sense. And you need that clause to be passive voice in order to keep the subject the same for both the main sentence and the subclause, especially if it isn’t important who did the insulting. (“The ambassador, who had been insulted, returned home…” also uses passive voice in the clause, by the way.)
Sometimes you really want passive voice for structural reasons. “The necklace had been stolen four times: once by the dwarves, twice by the elves, and once by the Queen of Siam.” has to be passive voice in order to get that nice parallel structure. You certainly could say “The dwarves stole the necklace once; the elves stole it twice; and the Queen of Siam stole it once” but the sentence doesn’t read as well and loses the build-up.
Finally, there are times when nothing but passive voice will do, and that’s all there is to it:
“‘He was murdered!’ the detective said.
“We don’t know. The only clue is this manuscript, which contains no trace of passive voice. You don’t suppose…”