Why does the US government carefully monitor horizontal mergers?

The US government carefully monitors horizontal mergers in order to prevent monopolies from forming. A monopoly is when one company or a very few companies control a particular industry. When companies merge and monopolies begin to form there is less competition in the marketplace, which means companies can set higher prices than if there were several companies all competing in the industry. Lack of competition is bad for consumers because it leads to high prices and a lack of variety in choices for goods and services. If there is less competition there is less incentive for companies to provide quality goods and services at reasonable prices. The US government monitors horizontal mergers carefully to ensure they will not unduly affect competition and therefore the consumer.