True, I don't know,

I just sit around in my room playing bass for 2 hours improvising my ass off and writing down the kickass stuff...

some people can't do it, I dunno why, but it's just a matter of constant improvising and thinking ahead, just think 'what kinda riff do I want?' and play it, lol, and from there on improvise it, that's the way I've done it for a long time now and it works fine for me.