Why was the road crossed by the chicken?
When a sentence starts with the thing being acted upon rather than the thing doing the acting, that sentence is in “passive voice.” For example, if you ask me where the hat is that you lent me, and I reply, “It got lost,” I have used the passive voice. A more honest and direct answer, using the active voice, would have been “I lost it.”
Active voice lends credibility.
In fact, passive voice is often used in order to avoid responsibility. “Mistakes were made” does not inspire trust and forgiveness the way “Our president made mistakes” can, especially if it’s followed up with specifics.
Active voice keeps readers interested.
In fiction, active voice helps keep the writing interesting and the reader engaged. “The front door was smashed by the speeding car” doesn’t grab the reader the way “The speeding car smashed in the front door.”
Sometimes passive voice is better.
A character’s personality comes through in his dialogue. Passive voice might show him to be a coward, a beaurocrat, or a hero-to-be who is about to grow in ways no one imagined.
Passive voice is also appropriate when you want to emphasize the thing being done, rather than the person doing it. “The tumor was completely destroyed by the radiation” is a fine way to give a patient the good news. On the other hand, if you were speaking at a conference for inventors of medical technology, you might want to stress the new treatment: “The radiation destroyed all traces of the tumor.”
As with the other writing principles you learn, avoiding passive voice is a generally good idea, but it’s not a rule. If anyone tells you otherwise, sent them to me. I’ll set ‘em straight.