Ever get distracted somewhere, like a grocery store, by some stupid little thing, and you realize minutes later that you had tuned out the entire world around you?
Or while buckling your child in the backseat, they drop something and you spend two minutes trying to find it; never mind the door is open behind you, the car is unlocked, and the keys are on the front seat where you left them?
A big part, nay, the BIGGEST part of self-defense is being aware of your surroundings. And keeping aware of your need to stay aware, even when something else happens that distracts you.
If you're in line to check out at Walmart, and you get a Facebook notification, it's not a good time to drape yourself over the cart handle and start scrolling. (I'm speaking to myself as much as you, here.)
Ever notice while watching a baseball game, that the only time something even remotely exciting happens, happens only when you look away for a second? It's the same case here. If you pay attention, you'll see stuff unfold and get early clues on how to respond. Or tips on when to exit the building in a speedy fashion.