If there are profits, more competition will enter the market and thus there will be more substitutes. If there are more substitutes the demand curve will become more elastic which will cause the price to fall and output to fall. The demand curve will rotate downwards which acts like a shift of the demand curve to the left. It will keep shifting until the demand is flat enough to be tangent to the ATC. Each firm will have a smaller share of the consumers than before. The demand for monop. comp. doesn't have the same determinants as a demand for the market. The determinants for monopolistic competition are entry and exit of firms, product differentiation, and advertising. For simplification we assume costs don't rise and we are in a constant cost industry.