According to HVAC professionals, there are many reasons why an air conditioner might run continuously, including frozen coils, inadequate airflow, and high outside temperatures. Contact a professional to troubleshoot a specific system.
One reason an air conditioner runs constantly is that the thermostat is set too low for the outside temperature. An air conditioner can only effectively cool outside air by 20 to 30 degrees, so when the outside temperature is over 100 degrees, an air conditioner will run constantly if it is set to 65 degrees.
If the air conditioner's coils are dirty, they can freeze up. This leads to inadequate cooling and a continuously running air conditioner. The coils can also freeze when the filter is too dirty to allow adequate airflow.