I think you have marginal utility confused with marginal cost. For a given supply, as demand increases, that causes an increase in price. But obviously, increasing the price does not (usually) cause an increase in demand.
If we're rationalizing this with economic principles, I think we can chalk it up to "imperfect information".
Also demand is static regardless of price changes. Quantity demanded is what changes. Apologies for the pedantry but don't want anyone reading too much here and making that mistake on a econ test :)
