lal is right... if you're soloing over a progression and stick to whatever scale (say a major pentatonic scale), then technically each note is going to "fit" correctly (that is of course if you know your scales and are in the right key). However, an important aspect of music is creation tension, and then releasing that tension (or maybe not). You know? Music would be boring if everything "fit" fine. Tension can be a good thing... this becomes ever more clear to me as I begin to learn about jazz... a jazz player probably would not play a simple g major scale over a g chord, but would spice things up a bit.