The essence of sound shifts: a conceptual question.

Listers, forgive a humble laic friend, and let me ask just a conceptual doubt. What is the essence of a sound shift. I'll try to explain: let's suppose there 's a word <sak> in a hypothetical language, meaning "river". People say <sak>, everybody knows understand its meaning. Why will any person or group start to say, for example <sag> or <zag> or <sok>? I can understand situations when there's different strata. I understand that a French or a Portuguese will not pronnounce easily the English <th> or the German <ich>, but when there's not a contact between two layers of languages it's hard to understand shifts. It's also understandable that different synonyms substitute other ones, but not the shift of sounds. Is ther any hypothesis to explain the universal and continuous sound shifts around the world?

Joao S. Lopes