There is an elusive and tricky balance that works back and forth between what we can feel and hear in the music inside ourselves, and what is learned intellectually and mechanically. Think first of ordinary communication. If you feel anger or sadness, your voice and body language will convey the feeling without you having to learn a technique that says: anger - expel a greater amount of air forcefully while tensing vocal muscles; sadness - lower voice, slow down speech. But physically we do these things to express those emotions. An actor, otoh, also learns to do such things deliberately, to be self-aware and observant. An actor is also using his body as well do in life. We are doing physical actions on an instrument, which is less direct.
So you're at the piano. Maybe you've experimented and you can make notes quieter and louder. So that same instinct comes as in speech --- sad = quiet and slow; angry is loud and harsh. You can put that in for a first kind of expressiveness. Even with this, however, I would want to know how to play loud and soft, to do staccato and legato. Or in the least hope that my teacher would monitor what I'm doing, watch that it's going in the right direction and give better alternatives if not. If you feel strong emotion, you can just as well end up being overwhelmed with emotion, and unable to turn that into any action that conveys it to your listener. Nobody can hear your wildly beating heart - you have to play.
I first played piano self-taught decades ago before taking it up again recently. What I did for expression wasn't necessarily good. I heard a crisp staccato in my head, and I tensed my arms and did a pecking-poking kind of thing. It made the right sound, but stopped everything else. How do you do fast staccato with tight arms? We need technique, which is the efficient way of using the body interacting with the instrument. That is a teacher's role.
Somebody mentioned theory. Well, when you understand the music this also tells you how to make it expressive. You recognize a phrase, and maybe make it climb in dynamics, becoming louder and softer. Maybe there are three phrases, and each will be louder up to a climax. (And then you need the technique to build that climax). Or playing one hand soft, one hand loud - and learning the coordination to bring that about. Or understanding meter and pulse - such as the strong beat in a waltz. Theory (understanding) and technique go hand in hand, because in theory you get a sense of what to bring out, and in technique you find out how to.