<human>: Can you condense the text into a few key points?

I see so many posts on Reddit (literally dozens a day) where people have found something out by checking (or "accidentally opening") something on their partner's phone. I did this not too long ago and I felt terrible after I did it - both because my suspicions were true and because I'd invaded my partner's privacy. But it seems like everyone is doing this. Are relationships so fragile that we feel the need to constantly check on our partners? And if so, given that trust is supposedly the biggest thing in a relationship alongside communication, why is everyone with someone that they seemingly don't trust?
<bot>: How many relationships involve the sneaky act of checking a partner's phone, and why stay with a person you obviously don't trust?