Being in the mood to write, like being in the mood to make love, is a luxury that isn't necessary in a long-term relationship. Just as the first caress can lead to a change of heart, the first sentence, however tentative and awkward, can lead to a desire to go just a little further.
The Right to Write: An Invitation and Initiation into the Writing Life (ed. Penguin, 1999) - ISBN: 9781101127834