Guideline. Unless you have a good reason for doing otherwise, express verbs in the active rather than the passive voice.
A verb is in the active voice when its subject is also the performer of the action. It is in the passive voice when its subject is something or someone other than the doer of the action. While there is nothing grammatically wrong with either construction, expressing verbs in the active rather than the passive voice usually produces sentences that are smoother and more concise.
Its drawbacks notwithstanding, the passive voice has several legitimate--and even preferable--uses. When a verb is expressed in the passive voice (as it was just then), the emphasis shifts from the doer of the action to the object of the action. Here are three reasons you might want this shift to occur:
Source: Grammar for Smart People by Barry Tarshis.