What are Chaucer's motives behind his peculiar shifts in narration in The Canterbury Tales?

Chaucer's motive was most likely just his own will to be a
dynamic story teller. Chaucer is frequently regarded as the Father of English
literature. The Canterbury Tales were presented in a brand new
format never before seen in English literature. Boccaccio's
Decameron does however resemble The Canterbury
Tales, as they both have a frame that connects their multiple stories, but
Chaucer's work is much more full and polished. In Chaucer's time, the norm was to only
base your stories after stories previously created, Geoffrey Chaucer had no problem
breaking free of that norm, as evidenced through the tales. He encompasses nearly every
aspect of medieval England, starting with nobility, see the Knight and the Squire,
followed by the clergy, see the Monk and the Prioress, and finally the middle class, see
the Wife of Bath, the Miller, and all the rest.

Chaucer was
well positioned and well favored in his time, and his home was strategically located in
an area where he would have gotten a very in depth look at the lives of all these types
of people. His decision to take on the tone of each character, and his reflection of
their own personality's within the actual stories was
ingenious. 

His transitions between the individual stories
are also very interesting, every prologue sets up a certain dynamic among the pilgrims,
allowing individual relationships to form, making the stories themselves more
interesting. Chaucer could just have easily decided to do an anthology of stories about
these characters that had no connection whatsoever to each other, but instead he builds
the reader's interest by exposing character flaws and poking fun of the corruption of
the time.