The price of art ? - Peter Doig example

Acclaimed artist Peter Doig is reported to have sold one of the most expensive paintings from a living artist. Originally he sold the work for a welcome £1000 to a collector. The painting was later sold for a staggering £7million. (dont quote me)
How does he feel? Bitter that he has not seen the monitory benifit? Flattered?.
My question is. "How can a painting increase in such value" ?
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