Learn Spanish (Latin America): A ella le dio por obsesionarse con la astrología y el tarot. - She has taken to obsessing over astrology and the tarot.

The structure darle por + infinitive expresses that someone suddenly started doing something on a whim. The verb dar is conjugated in the third-person singular (here, dio in the preterite) because the grammatical subject isn't the person. Instead, the person experiencing this sudden urge acts as the indirect object, which is why we use the pronoun le (clarified by A ella).

Why is the pronoun se attached to the end of obsesionar?

The verb is the pronominal obsesionarse (to become obsessed). The pronoun se must match the person who is obsessing—in this case, ella. Even though she is the indirect object of the main verb dio, she is the logical subject performing the action of the infinitive, so the pronoun reflects her.

Is the preposition con always used with obsesionarse?

Yes. When you want to say you are obsessed with or over something in Spanish, the verb obsesionarse strictly requires the preposition con. You cannot translate the English "over" or "about" literally here.

Why are the definite articles la and el required before astrología and tarot?