From Kenneth Koch and Kate Farrell’s Sleeping on the Wing:
Suppose you want to get an experience into words so that it is permanently there, as it would be in a painting—so that every time you read what you wrote, you reexperienced it. Suppose you want to say something so that it is right and beautiful—the way music is right and beautiful—even though you may not understand exactly why. Or suppose words excite you—the way stone excites a sculptor—and inspire you to use them in a new way. And that for these or other reasons you like writing because of the way it makes you think or because of what it helps you to understand. These are some of the reasons poets write poetry.