Conversation so far:
what was Norman Rockwell's most famous painting?
His Sharp Harmony appeared on the cover of the issue dated September 26, 1936; it depicts a barber and three clients, enjoying an a cappella song.
what was the theme of most of his paintings?
cartoonist.
for how many years was he actively painting?
Rockwell published 323 original covers for The Saturday Evening Post over 47 years.

Question: why did he stop painting?
Self-contained rewrite: why did Norman Rockwell stop painting?