AOO is set to All Enemies by default. Oh wait. I'm in a battle against Ragzlin and like a dozen cultists. On Lae'zel's turn, I switch AOO to only trigger on Ragzlin because he's the only person I want her to AOO against of every enemy near Lae'zel. From that moment on in that battle, Ragzlin is the only target she will AOO until I switch the preset again.
Yeah, and my counter-example of why I wouldn't want that is this: I set my AoO to work only on Ragzlin because I consider him the priority target. During his turn Ragzilin does absolutely nothing to activate it, let's say he even moves first and just attacks on the spot. I'm left for the entirety of the remaining round wasting all my AoO opportunities while two other goblins could have triggered it.
So... I wasted more time that confirming a prompt, I had to make a decision upfront that involved switching toggles, selecting targets and what else; in the end it amounted to absolutely nothing. Why should I be happy with this system, again?