To answer the reason why I write, I'd like to answer why you, me, and everyone else writes, to communicate. Whether you are writing a poem, short story, or play you are communicating to others by sharing opinions, feelings, moods, thoughts, facts, and ideas.
The reasons you and I write can be different because we may be trying to achieve different things with our writing such as congress trying to pass a bill, everyone has there own idea and opinion of how something should be done and is trying to persuade others why their way makes more sense than the other guys. An encyclopedia is written to inform you with information or like fiction books are mainly written to entertain. Even a diary written to yourself is communicating because you are writing and reminding yourself what you had done on that day and what your feelings and thoughts were.
So to answer the question of why I write, I write to communicate and interact with others to get a point across, entertain, or persuade. And I set the mood or setting by describing emotions or feelings when i'm entertaining and facts and theories when i'm trying to persuade or inform. So in short you, me, and everyone else writes to communicate or interact with each other.