I’ve had issues when partners have had issues with trust and/or control.
Once I was having dinner with my mom, and didn’t reply to a text for 30 minutes. The next text was “This is why I can’t trust you.”
In another relationship, she didn’t trust me and thought I’d been with someone I hadn’t before we were even a couple, never believed me about that, thought I was flirting and staring when I wasn’t, etc. I was quite loyal for years but eventually couldn’t take it. Needing to know where I was at all times, questioning how long it took me to get home from work, etc.
I think privacy gets related to trust, openness, and control issues. As an introvert, I have a certain amount of need/desire for time alone, and to feel free to spend some time and do some things and have some communications without feeling like I’m being observed or judged. There’s a huge sense of freedom to be myself when I’m alone and unobserved. The more trust and understanding there is, the less I need/want that. It’s not that I have something to hide; it’s that I’m aware of being observed and how it affects my choices and experience.