preocupar vs preocuparse
Preocupar means 'to worry someone' — gustar-style. Preocuparse means 'to worry yourself'. Both are common and easy to mix up.
preocupar
To worry, concern someone — the cause is the subject.
Me preocupa tu salud.
Your health worries me.
The cause is the subject. Gustar pattern.
¿Qué te preocupa?
What's bothering you?
Asking what's on someone's mind.
No me preocupa el dinero.
I'm not worried about money.
Negative: not a source of concern.
preocuparse
To worry, to take care or be concerned about someone — you are the subject.
No te preocupes.
Don't worry.
The most common use. Command to relax.
Me preocupo por mis hijos.
I worry about my children.
Preocuparse por/de + person or thing.
Se preocupa demasiado.
She worries too much.
General trait.
Tricky bit
'Estoy preocupado' (I'm worried) is the state. 'Me preocupo' is the habit or the act. And 'preocuparse de' can mean 'to take care of': 'yo me preocupo de la cena' = I'll see to dinner.