How do you create a guitar riff?

Well, pretty much the easiest way to do so is to just doodle around on the guitar until you figure out something new and fun to play. That's the essence of it all. However, knowing what sounds good with what else (music theory, chord patterns, circle of fifths, pentatonic scales, etc.) really helps in creating something that sounds good right off the bat. The problem is that everyone else can learn the exact same things, so making something that is new and unheard of is increasingly harder to do.