Do you think the Mona Lisa has a little piece of paper from the artist stating:
This piece it to demonstrate the the socio-economic trends of the day. This the smirk showing the arrogance of the upperclass over the lower...
Or how about the statue David?
This statue is made of a large size to repressent Roman culture's on going over bearing influence on western culture. And everything is anatomically correct except the genitalia, which is small, repressenting the impotence of the Roman Empire and how it fell to the baser barbarian hordes...
No? You didn't see those artist notes either?
It's like humor. If you have to explain a joke, it's not a good joke.
For me, I think, "because it's pretty," is all the explaining ever needed.