I'm reading, writing, or playing a computer game. I'm in the middle of a really good, high-emotion, climactic scene and my entire world has become the artificial creation that commands my attention.
Then someone calls me and takes me away from the fantasy.
This drives me to distraction. Of course, it's not the person's fault for calling. They have no way of knowing that I'm right in the middle of finding out how Harry Dresden gets out of his latest dilemma, or that my protagonist is about to realise what's really going on behind the scenes. And most of the time it's my heavily pregnant wife calling, who I'm all too happy to help out, whether she needs me to make a cup of tea or scratch part of her back she can't reach. But still, it's a tragic moment when the illusion is shattered.
Wouldn't it be great if we could broadcast an all-points "do not disturb" bulletin?
Of course, my husandly duties require that I do whatever my wife needs, because I'll be dammed if I let her hurt herself doing somthing that I could do for her easily.