Explain why a competitive market would generally lead to too much output and consumption of the goods if the production of the good generates a negative externality

The market in a competitive market attains equilibrium where the marginal private benefit is equal to the marginal private cost. In the case of negative market externality in production or consumption of the good, a marginal external cost is exerted to the society at large. This makes the marginal social cost much greater than the marginal private cost. Since the market fails to incorporate the marginal external cost from production or consumption of the good, the market tends to overproduce the good. The socially optimum level of production for the good is much less than the output produced by the market, and comes at a much greater price than the market equilibrium price. This is the reason why the market tends to produce too much output over what is desired by the society.