Because I do not live in the 20th century, I keep photos of my baby on my phone, not in my wallet. Which is a problem: While you probably wouldn't rifle through my wallet, you happily invade my phone. Dick.
To wit: Often when I show someone a picture of my kid, they immediately begin scrolling around, looking for others. Okay, sure: She's cute. And I've got hundreds. But that's some serious bullshit.
When someone hands you their phone to view a picture, they're really handing you their trust. They're handing you permission to see one particular thing. It's not like you have to fire up their email to violate that. All you need to do is flip just one photo over without asking.
When you start viewing other photos without permission, or God forbid, exit their library and peruse their apps, it's an invasion of privacy just as surely as if I let someone take a piss in my bathroom and they used that as an invitation to poke through my medicine cabinet, looking at my pills and crémes.
Uncool! I do not want you knowing about my Xanax habit and hemorrhoids. Metaphorically speaking.
Our phones are just as personal—probably more so—and it goes way beyond the photo album. They are our diaries, our photo albums, our financial planners, our therapists. They have the potential to reveal our most damaging, or at least embarrassing, secrets.
We keep apps on our phones to help us fight our addictions, live with deeply personal health issues, meet anonymous sexual partners. Our libraries have naked photos of ourselves and our significant others (or even more damagingly: people who are not our significant others.) You can ruin someone's life by idly flipping just one screen over from the one that was handed to you.
Again, this is why you ought keep your phone locked, and teach others to do the same.