S: what makes a paiting classic?

One of my favorite current artists is Ron Monsma, an Indiana-based artist specializing in Oil and Pastel. He's brilliant, really--the website doesn't even fully showcase his talent, but you can see his eye for texture and light. His Pastel landscapes are almost Monet-esque, and I think his beautiful, rich still life paintings could easily be taken for ones by the 'old masters'. Which leads me to wonder: what makes a painting 'classic'? Is it just age? Does the artist have to be dead in order to be appreciated? Why is it that a painting is automatically treasured because it is old or by a famous artist, while one that is better never gets noticed? Or is it because classical artists are just so undervalued now because of the modern art craze? I appreciate modern art in its own right, but one only has to look at a painting like this to realize the talent that goes into it.

Thoughts? Happy Sunday, yes?