So this is timely:
http://www.guitartricks.com/forum/showthread.php?t=39566

@TravisWrite - One could probably write a dissertation on the topic :-). The post above (which I haven't read all of the way through yet) hits this head on.

What else contributes? Just throwing things out there... your current mood, your life experiences to that point, the impact of the people around you have on you. Look at Miles Davis' life and how his musical style changed and varied throughout his career. He was hugely influenced by the people he surrounded himself with.