A watched pot never boils does not have an express origin. It is called an idiom. It means something else besides what it implies.

A watched pot never boils. Submitted by "the Gatling's"

Be patient. The things you desire will come to fruition in their own time.

Conan - 2010 A Watched Pot Never Boils and Other Total Lies 2-41 was released on:

USA: 30 January 2012

Finland: 7 February 2012

it means that not being patient can make time seem to slow to a crawl therefore a watched pot will never boil