In micro-economics, why is a demand curve downwards sloping?

A downwards sloping demand curve in a supply and demand diagram for micro-economics sits on a X axis named quantity and Y axis named price. The demand curve is linear and slops downwards start from Q=0 and P=X. A rational consumer can conclude that on any given point along the demand curve, as you move along it, quantity increases as price decreases, which is expected of consumer behaviour.However, economists have gone on to say it is also because of marginal utility. Utility is the total satisfaction received by the consumer, and the maximum price the consumer is willing and able to pay represents this metric on the demand curve. The theory suggests that after reaching a certain quantity, the consumer will no longer be as satisfied as their first purchase, hence a decrease in utility, meaning a decreasing marginal utility. Hence, the more a good or service a consumer uses, the less marginal utility per additional unit is received, resulting in a downwards sloping curve.