Without context I'm tempted to say focusing is about a whole bunch of things and that telling people to say no is just another way of saying, 'Use your willpower.' Which is another way of saying 'Focus by focusing!' Which... seems rather recursive at least.
One of the things that focusing is about is giving up pursuing good things.
Which means that if I want to focus, I need to decide which good things I'm going to say "no" to.
This may seem obvious, but after seeing many not-otherwise-stupid management structures create lists of "priorities" that encompass everything good (and consequently aren't priorities at all), I'm inclined to say that it isn't as obvious as it may seem.
Another monthly installment of the rationality quotes thread. The usual rules apply: