I agree with the comment regarding Chaucer as a great observer. As we all know, any person with a vast experience within life is typically the person that can best retell "life."
Chaucer, whose experiences in life range from "as a lad, in the service of Elizabeth, countess of Ulster and wife of Lionel, the Duke of Clarence" to one an esquire for King Edward III and later Richard II - definitely had the life experience.
Yes, Chaucer was born to a middle class family, but middle class during that time was nothing more than a class of paid traders. His family (possibly known as shoemakers) had established themselves in London. Remember that the middle class was established during the time period of Chaucer due to the increase of unpleasantness among Peasants. Therefore, Chaucer's experiences as a child were probably very similar to that of a peasant in prior years (which is probably why the countess had him on her payroll at the age of 12).
Moving through the ranks as a young adult offered Chaucer a chance to develop opinions and observations that would otherwised have gone unnoticed if he had not grown up in the "middle class" of London.
His work experiences throughout his life basically make him an expert on the workings of the Church, the Monarchy, Parliament and the Commoners.
Today, Chaucer would be looked to as a phenomenal social broadcaster. My senior English students often compare him to the host of The Soup or TMZ episodes in which some guy provides commentary on the actions and experiences of celebrities or high profile people within our society.
In class, I often present Chaucer as the instinctual and psychological observer - he almost knows what the person is going to do before the character does (oh yeah, he did create the character, didn't he?).
As far as his mastering and basically the invention of this technique (Boccaccio fans will object) sit down and write about the 10 people you know best in your life and see if you could create an archetypical persona...I bet you can!
Part of Chaucer's genius lay in the fact that he was a great observer of society. Born into a "middle class" family, he grew up with the milliners and tradesmen. His brilliance was recognized early on and was sent away for his education. This of course, exposed him to the clerics and intellectuals of England. He went on to work as a squire in the court of Elizabeth, Countess of Ulster, giving him further insight into both the royal and servant classes. (The third link below is helpful with Chaucer's background.)
Besides Chaucer's observational skills, he was also extremely intelligent and was able to draw upon his observations to create accurate characterizations--not only for individual characters, but for what they represented to society.
The Canterbury Tales are not only one of Britain's greatest contributions to western literature, but Chaucer's skills also allow the text to provide insight to life in the 14th century.