Learn Spanish (Latin America): Es un interés repentino; ya que quiere mejorar, le dio por ayunar. - It's a sudden interest; since he wants to improve, he took to fasting.

The structure darle por followed by an infinitive expresses a sudden, often unexpected or capricious urge to do something. Here, dio is conjugated in the preterite (past tense), and the indirect object pronoun le tells us who got the urge. Literally, it's like saying "it gave him the whim to fast."

Why use the indirect object pronoun le in le dio por instead of a subject pronoun like él?

In this expression, the sudden urge is treated as the grammatical subject (the "it" that does the giving), and the person experiencing the urge is the receiver. Because the person is receiving the action, we must use an indirect object pronoun like le. If the sudden urge happened to you, it would be te dio por ayunar.

Could I use como instead of ya que to say "since" in this sentence?

Not in this exact position. Both ya que and como can translate to "since" or "given that." However, when como means "since," it is almost always placed at the very beginning of the sentence (for example: Como quiere mejorar, le dio por ayunar). Because the "since" clause comes after a semicolon here, ya que or porque are the most natural options.

Is the adjective repentino related to the phrase de repente?