The main sources in The Canterbury Tales comes from the different aspects of irony used by Chaucer throughout the text. Chaucer uses verbal irony, situational irony, and dramatic irony in the text. It is this use of irony that provides the main source of humor throughout the text.
For example, in "The Pardoner's Tale", the priest constantly advises against the avarice (or greed). But, instead of taking his own advice, he is guilty of avarice. By allowing people to buy pardons and relics from him, the priest is lining his own pockets. This is an example of situational irony. Situational irony exists when the opposite of what is expected to happen happens.
Another example of irony in the story exists, again, in "The Pardoner's Tale". Here, the irony depicted is one of dramatic irony. The three thieves, on a quest for Death, find a bag of money. Each one decides that it would be in their favor to not have to split the wealth three ways. The men make plans to do away with another. In the end, all men die. This is an example of dramatic irony given the reader knows more about what is going to happen than the characters involved. The reader recognizes that when one seeks out death, the person will find it.