Deponent verbs are the bane of the young Latin students existence. They take the form of the passive voice, but they have active meaning. And they are darned common: loquor, I speak; confiteor, I confess; morior, I die. Many of them are transitive verbs, and so they can take an object where the look of the verb wouldnt suggest any. Why would the Romans have such a ridiculous thing?
These verbs, though, really do occupy a middle space between active and passive. They are like Greek verbs in the middle voice, in which the subject is both acting and acted upon. Consider these sentences:
I hurt the quarterback.
I was hurt by her remark.
Notice the differences between the three? In the first, the true active voice, the subject is the agent of the verb. In the second, the true passive, the subject suffers the action of the verb. But in the thirdwhat? The subject is the agent, because hes actively experiencing something named by the verb; but he suffers the verb. Im hurting does not mean I am walking around the neighborhood punching people, but I am feeling hurt; something is hurting me.
Most of the deponent verbs are of this sort. Consider: morior, I die. Im agent and patient at once. Consider: sequor, I follow. Again, Im doing something; but something at the same time is being done to me: I am made to come after someone else. Consider: fieri, to become. That may be the definitive active-passive verb: when A becomes B, A is doing something: and something is being done to A.
Language isnt always irrational, you see.