Nightowl,

Yes, there is a simple explanation, and that is that the G7(G-B-D-F) chord has tension in it that can be resolved only by moving back to the C (C-E-G). The tension is a strong dissonance between the G and the F in the chord.

If you don't understand what I mean, just play the two chords--Play C, and it sounds like you can stay on C--Play G and it sounds like you need to go somewhere.

This tension/resolution is a basic element in music. In classical western music(and in western folk music), we resolve it in by moving back to the fundamental (C).

Some composers a long time ago liked working with musical tension in this way, and so almost all of our music is built around it--it isn't the only way you can do it, and other musical traditions do things differently(though they still work with the tension/resolution thing).

That is music theory.