One of the first questions about writing is "why write?" There are many reasons to write, of course. You have to write that thesis if you want that degree. That piece of equipment needs a manual so people will know how to use it so someone is given the task of creating one if they want to be paid. And those texts, emails, and, yes, blog posts won't write themselves.
Here, however, I'm going to focus on fiction writing. Why take the time and effort to sit down (or pace up and down the hall, or however one does it) and craft a story?
So what are the reasons for writing? Fame? (Twenty years writing fiction with a handful of professional sales so far and I'm still pretty much unknown. There are easier ways to fame.) Fortune? (The most I, personally, have made in a year from writing is about $1000.) Attracting members of the appropriate sex? (Have I got some disappointing news for you....) You get a few, a very few, who are successful by those metrics, sure, but mostly you get folk who labor away for a little it of extra pocket money (or a modest living if they're lucky) or the occasional fan letter. (Science fiction and Fantasy have an advantage over many writing genres in that there are frequent conventions where fans of their style of writing get together and authors and fans can meet each other.)
So why do it? It's a lot of work for very little of the typical rewards.
One thing to remember is that the storyteller is as old as humanity itself. Telling--and with the invention of writing, writing them--is just something people do. Even traits that are ubiquitous across the human species are expressed more strongly in some than in others. Some have a stronger drive to tell stories in much the same way that some people have a stronger competitive drive.
In the end, I think that you aren't a writer because you write. You write because you are a writer. Making money, winning fame, making friends and influencing people are often rationalization more than reasons, a justification for the mental and emotional effort that goes into writing.
That said, writing, storytelling, isn't the only drive and, as drives go, it's fairly far down on the totem pole. Yes, I have a drive to write, to put stories down for other people to read, but I also have a drive to eat, to live in reasonably comfortable surroundings, to procreate (and the things that go with that), and so forth and if writing gets in the way of that, so much the worse for writing. Other people might have the drive more strongly and are willing to live a hermit's life in a drafty attic somewhere while scribbling away the story they have to get out of their system only, once finished, to have it replaced with another story that they simply have to get out of their system.
And so, while I think to a great extent writing and storytelling is what you are more than what you do that it's not also a craft and a skill to be learned. There's a big difference between a group of friends telling "no fooling, there I was..." stories at the local watering hole and someone writing a novel that sells hundreds of thousands of copies. Some of that difference is just plain luck. Some of it is inborn talent. And some of it is learned craft. I'll look at that a bit another time.