The only music capable of moving and touching us is that which flows from the depths of a composer’s soul when he is stirred by inspiration. There is no doubt that even the greatest musical geniuses have sometimes worked without inspiration. This guest [i.e. inspiration] does not always respond to the first invitation. We must always work, and a self-respecting artist must not fold his hands on the pretext that he is not in the mood. If we wait for the mood, without endeavouring to meet it half-way, we easily become indolent and apathetic. We must be patient, and believe that inspiration will come to those who can master their disinclination.
Pyotr Ilyich Tchaikovsky
(read the full quote on Brain Pickings)