Why the verb is uesd as active voice for the thing ?

Hello everyone

I have seen many sentence using active voice for things, For example :

The pen was stolen (passive).
The price is rising (active).

I wonder why in English some verbs can be used in sentence as active voice for things. Teachers teach me we should use passive voice if thing is a subject of the sentence.

Some things can act, that is all: prices can rise, but pens cannot steal. You will have to use common sense.

Thank you Mister Micawber.