There are, at least, three (3) must-haves of every writer:
Per James Baldwin a writer must cultivate the state of being alone
The primary distinction of the artist is that he must actively cultivate that state which most men, necessarily, must avoid: the state of being alone. - JAMES BALDWIN
Per Joyce Carol Oates a writer must be able to concentrate for long periods of time
First requirement of the writer is the ability to concentrate for long periods of time. Second, more urgent requirement, the wish to do so. - @JoyceCarolOates
And Malcolm Gladwell opined that a writer must have have more than an engaging story but that a writer needs to have a strong desire to tell the story
When you write a book, you need to have more than an interesting story. You need to have a desire to tell the story. You need to be personally invested in some way. - Malcolm Gladwell