"If I create from the heart, nearly everything works; if from the head, almost nothing." ~Marc Chagall
Artists give good advice. I agree wholeheartedly here with Chagall. One look over his paintings and you can see they came from love, an innate love not only for what he was painting, but for life itself, and love.
The same applies to writing too, of course. If you don't love your subject, it will fall flat in front of you. The head is fine for fixing and editing and analysing, but the creation point, the will to create, the motivation, the spark of inspiration, must come from the heart, the site of feeling.
Science-fiction writer Ray Bradbury has echoed the same sentiment:
“Write only what you love, and love what you write. The key word is love. You have to get up in the morning and write something you love, something to live for.” He then goes on to instruct us budding writers: "...And out of that love, remake a world.”
Ambition won't do it, talent alone won't do it. Only love - love for what you do, pure passion, innate investment - will. As Bukowski too, puts it - write only if 'the sun inside you is burning your gut' to do it.