気になる vs 気にする — which one is the intransitive/uncontrollable one?

Question

I keep mixing these up. 「彼のことが気になる」and 「彼のことを気にする」both seem to mean 'I'm bothered by/thinking about him,' but one takes が and the other を. Is there a real difference in nuance, or mostly just grammatical structure?

Accepted Answer

気になる (with が) is the spontaneous/uncontrollable one — something naturally weighs on your mind or catches your curiosity without you choosing to think about it; it's an intransitive state, closer to 'X is on my mind.' 気にする (with を) is the deliberate/willful version — you actively choose to worry about or pay attention to something, and it's often used in the negative to tell someone NOT to dwell on something: 「そんなに気にしないで」= 'don't worry about it so much' (you're telling them to stop the deliberate act of worrying). So 彼のことが気になる = 'he's on my mind' (just happens), 彼のことを気にする = 'I'm worrying about him' (an active, ongoing concern I'm engaging in).