Why does Chaucer represent the characters in The Canterbury Tales both negatively and positively?
Let us remember that Chaucer's key intention in writing these tales is to entertain. Therefore he paints the various characters that join together on this pilgrimage in all of their glory, both focusing on the positives and the negatives, which, if we are honest, any audience might find more humorous and appealing. If we examine the General Prologue therefore, we can see the way that Chaucer, who casts himself as the narrator of this piece and as another character on the pilgrimage, is rather ironic in the way that he introduces the various characters, focusing both of their positives and negatives. For example, consider the presentation of the Friar:
He was a noble pillar of his Order,
And was well in and intimate with every
Well-to-do freeman farmer of hsi area,
And with the well-off women in the town...
He was an easy man in giving shrift,
When sure of getting a substantial gift.
We have the ironic contrast of the Friar being "a noble pillar" but then also clearly being out for all he can make from rich people. If you want an example of a character that is presented favourably, consider the Knight:
There was a knight, a reputable man,
Who from the moment that he first began
Campaigning, had cherished the profession
Of arms; he also prized trustworthiness,
Liberality, fame, and courteousness.
This is definitely a character that is spared a lot of Chaucer's mocking commentary, though he is made out to be rather dull and so focused on chivalry that he lacks the flair and colour of the other disreputable characters.
This collection of tales therefore presents both the good and bad side of characters because it sets out to entertain, and I think in a sense it is the "bad" characters that are the most vivid and real and offer the most entertaining stories in some senses.