というのも
the reason is, you see
Statement。というのも、reason + からだ / のだ。
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というのも gives the reason behind what was just said: 最近、よく眠れない。というのも、隣の工事がうるさいからだ, "I haven't been sleeping well lately. The reason is that the building work next door is so noisy".
It's like なぜなら (N3), but softer and more conversational. The reason usually ends with からだ, のだ or んです.
It sounds like an explanation offered to help the listener understand, rather than an argument. That makes it common in explanations, apologies and storytelling.
Don't confuse it with というのは (N3, "what X means") or というと ("speaking of").
Examples
最近、よく眠れない。というのも、隣の工事がうるさいからだ。
I haven't been sleeping well lately. The reason is that the building work next door is so noisy.
今日は早く帰ります。というのも、母が入院しているんです。
I'm going home early today. You see, my mother is in hospital.
彼女は怒っている。というのも、私が約束を忘れたからだ。
She's angry, because I forgot our plans.
旅行はやめた。というのも、お金がないからだ。
I've given up on the trip. The reason is I don't have the money.
彼に頼んだ。というのも、この分野に詳しいからだ。
I asked him, because he knows this field well.