If we all kept attention every single minute of our lives,
we would all be smart.
But in reality not even the smart keep attention every single minute of their lives.

And sometimes, if you aren't concentrating, you are bound to have problems later.
So you just have to choose the time and the place to focus on.
That's pretty hard, isn't it?
And there's no rational way to know these things.
Also because they don't really make sense.