I write for myself. My composing style happens to have a lot of Beethoven influence, but still, I write for myself. As for whether I write in specific forms or just let the melody lead me, I do both really. I have pieces like my Scherzos, Waltzes, and Sonatas where I go for a specific form. And I have pieces like Adagio in E minor, Tears of the Sky, my Requiem for String Quartet, and many more where the melody led me to the rest of the piece.
Even in my form based pieces though, the first theme at least is usually just a melody that I improvised on the piano and then harmonized.