Well, Joe, art is different today than it was in antiquity. In ancient times art was considered to reflect the mystery of the human soul and thereby artists considered the "ideal" to be standard in their compositions (for example, the marble statue "David" by Michelangelo). This canon reflected the society's values, stories and sometimes daily life scenes in which took place contemporaneously.
Throughout the centuries, artists have struggled to be seen as noblemen, not just mere craftsman of their work. By the 1800's the Academy was formed and was the catalyst for each artist to achieve individually in their art. That brings us to the modern art movement (1920's - 1960's), where art now was seen as an exploration of aesthetics and medium.
Art changes throughout the centuries, in my opinion, because an artist as a human being naturally strives to discover further opportunities, to discover new things, and sojourn to illuminate what was previously unseen by others.
Art always reflects society; an artist's surroundings are their inspiration.
Hope this helps! *-*
