Inspiration is a burn inside certain people. Not all people have it, at least not in the vein of writing. How it got there is very intangible, but that it is there, is undeniable.
It is a drive, like the overwhelming desire certain people have to be a pro athlete. It is a hunger, like the ache that others have to be missionary in 3rd world countries. It is unquenchable, like the emptiness that a childless woman has to be a mother. It is there, in and of itself. It can be ignored, but it will not go away.
Writers write. It's that simple, and that complex. Writers keep on moving ahead, regardless if they have sold a single piece, sold 10 best sellers, or have file cabinet full of semi-finished and completed works.
Writers see writing in everything they observe. They see the world in cross-section. Where the casual observer sees a single mom, the writer sees a victory. Where the casual observer sees a homeless man, the writer sees a former chess champion whose time is past. Where the casual observer sees a bus driver, a cashier, a policeman, a company owner, the writer sees a man trying to go back to school and get his high school diploma, a woman who had to leave a domestic violence situation, the woman's husband, and a man struggling with alcoholism and losing his family.
Inspiration imposes itself upon the writer, at the most inopportune times, and the writer can flow with it, can ignore it, but the writer cannot stop it. For the writer, inspiration will always be knocking and asking to be let in.