Why is the demand curve downward sloping?

So this may seem a bit odd at first - like why is it the the more of something there is, the less it costs to buy? But it actually makes a lot of sense when we break down the law of demand. The law of demand states that "ceterus paribus (which means if all other factors remain the same) as the price of a good increases, quantity demanded decreases; conversely, as the price of a good decreases, quantity demanded increases".Think of it this way, if pizza costs £10 and you’re buying it twice a week. If, all of a sudden, the price increases to £20, it makes sense for you to just buy one pizza a week, i.e. because the price has increased you buy less. Conversely, should the price fall to £5, this may been you end up buying 4 times a week because it costs you the same amount to have twice as many pizzas. As we’ve seen the diagram we use for demand and supply has quantity along the x axis and price along the y. Mapping our pizza example on to this diagram we see that at a high price we order less pizzas, so we’re in the top left corner side of the diagram, and as price falls we start to order more - therefore we move down and across the diagram and end up in the bottom right corner of the diagram. Ultimately this gives us a downward-sloping demand curve.