This sort of experience is fundamental to developing an appreciation for privacy. It just feels bad on a pretty basic level to have your privacy violated, or to uncover things you weren't supposed. Here's my two big transformative privacy moments.
I'm fifteen and talking online with various strangers from all over the world, as one did at the time, and one night at 3am one of my closest friends calls me as a joke. It was an older man from a different country. My mom answers, freaks out, and wakes up my dad who comes into my room and drags me away from the computer and asks who the hell is calling for me at this hour. I tell my dad it's a friend. He asks me who. I tell him his internet handle and my dad calms down, tells my mom it's okay, and sends me to bed. What I put together later and through similar experiences was that my dad had already vetted the guy. He knew everything I was doing and saying to people. Everything. It took me a long time to understand he was a nervous, technically-minded parent navigating a new aspect of parenting in the early 2000s. But the feeling of paranoia and violation lingered.
The second experience was being an insecure teenager and accessing people's emails and msn conversations. The things I learned were things I didn't want to learn, and weren't fair for me to know, and they just made me feel bad, untrusting, and slimy. But I'm grateful to have learned that early in life. I don't care about other people's emails or texts. I have zero interest or inkling to go through anyone's phone or data out of a basic level of understanding and respect for privacy.