If you think you are writer, I mean if you write, and look at the world with the head and eyes of a writer, you see the world through a unique perspective. The events of a life, the things that happen in a life, the feelings, the thoughts, the emotions, the coming and goings of people, and things, are just raw material for the writer.
This puts the writer in a writerly mode - a step back from the hustle and bustle, the hurly burly. If you keep your writer's eyes, your writer's head, you are always observing, looking for things that you can "use," that you can put down in words on a page.
There is a little arrogance, or maybe self-centeredness about the process. Not only is the writer experiencing the world, but he or she is already thinking how this all can be rendered into sentences and paragraphs. And how this can be explained, or depicted for others. It's a way of imposing a narrative, a line of thought on an endless stream of events.
Are there some things the writer shouldn't write about? It's up to the writer.
What is important? What stands out? What is interesting? What is extraordinary? It's up to the writer. If the writer is good, this can be astonishing, revealing, enlightening. Or it can be deadening, concealing, dumb-making. It's up to the writer. And the reader too.
Oh yeah, the writer imagines the reader too. It's kind of a self-enlcosed universe...