The materials used in the construction of your home contain moisture and will expel water over the course of a season. Basement garages exhibit more condensation as a result of ground water moisture.  Or in some cases freezers, washing machines, dryers and water heaters put warm air that contains moisture into the garage.
Creating more ventilation or using a dehumidifier in the garage may help with water condensation.  Upgrading your door from a non-insulated door to an insulated door may also reduce the amount of condensation.