You can write when you are not in the mood, but rarely does it prove fruitful. But if fruits are not what interest you, then I suppose it might be worthwhile to do it anyway. Such is the case for me today. I'm writing about writing. You might think that that would indicate that I have a dearth of material. You would be wrong. I could be writing about the US-China space race, the recent Palestinian-Israeli conflict, John Travolta's son, the EU's new President Vaclav Klaus, or about how I'm reading the sixth book in the Chronicles of Narnia series, but I'm not.
I'm writing about writing.
It can be entertaining (one of the main reasons I do it) or it can be therapeutic, as I mentioned earlier. Writing is essential for speaking well, reasoning well, and making sense of the world. However, while writing often helps the writer make sense of the world and his own thoughts on the world, the world still remains quite unpredictable, strange, and inexplicable to those who do not write, as well as it does for those who do write and do not agree with you. However, that is no reason to give up on writing.
As you increase your vocabulary and reasoning skills through writing, you inevitably must embrace new theories, ideas, styles, and roots. While doing this of course your mind hypothetically or even quite literally expands and allows you more room to find room for other people's ideas, cultures, styles, and foundations. This newfound attitude of openness however does not mean that a person need throw out all of their initial pretenses for ones that may seem more erudite and evolved. But, what it does mean is that a writer must be more willing to entertain and consider opposing views, and ideas. That consideration in and of itself is progress.
Yay for progress! And, yay for writing.