Anyone who has ever experienced the special kind of horror of having your mom, spouse, or coworker (God, it could be anyone) swipe through your photos and stumble upon a photo they definitely should not see will 100% agree with this statement: the next iPhone needs to have a guest-mode feature.
Even if this hasn't happened to you, God willing, we're all packing classified information of some variety on our phones. There's a certain anxiety that engulfs you whenever you hand your phone off to someone else, whether it's to give them a closer look at a photo or show them a hilarious text exchange. Are they the type of snoop who will take the liberty of scrolling past said photo? Will they accidentally get a peek at the steamy sexts you've been exchanging with your boss?
Conversely, you can always tell when someone is really, really reluctant to relinquish control of their phone. They hold it out across the table for you to see something, and you reach to take it -- then there's that awkward moment when you realize they don't trust you enough to let go.