There were several causes of the Great Fire. Historian Samuel Pepys reported that one of the royal bakers, Thomas Farrinor, neglected to put out the fire in his oven. Ordinarily, this kind of mistake would burn down the house where the fire started and maybe a few of the surrounding buildings as well. In 1666, however, London was extremely overcrowded. Many of the buildings were very old timber-framed structures. The roofs of these buildings were covered with pitch, a tar-based product used for waterproofing. With all the wood and pitch in the city, London was like a matchstick waiting to be struck. Visions of the apocalypse, especially considering that the year had the number 666 in it, were in many people's imaginations. But London survived the fire, and King Charles II appointed a commission to redesign the city, with wider streets and more brick buildings.
Visit the links below for more information. See especially the luminarium.org materials, which includes illustrations of the fire and a diagram of the areas in the city that were affected.
The fire started in Pudding Lane, and one of the royal bakers, Thomas Farriner, who owned a bakery shop there, his maid had forgotten to put out the oven during the night, so the tremendous heat created by the oven caused sparks ignited to the wooden house of his. Once the fire started, it spreads wildly and quickly as London was basically make up of wood to be honest. Also, if the houses nearby were demolished quickly, the fire wouldn't have spread so quickly.
London was overcrowded with timber-framed building structures, so London was basically a still time-bomb waiting for a right time to explode and create a human apocalypse and a humanitarian crisis.
After the disaster, King Charles II decided to redesign the whole city and appointed commissioners to create wider pavements and roads in streets and creation of buildings made up of brick, rather than timber, as it won't catch fire too easily. The Fire was a complete disaster, and luckily, there weren't too much deaths, except for the maid who forget to close the oven- poor her.