Water condenses to form cloud droplets when the air cools down to a point below saturation (100% relative humidity), also called the dew point temperature. Most often this happens when air rises. As a parcel of air rises it moves to lower pressure and cools due to expansion. If it rises and cools enough it will reach the dew point temperature and cloud will form. As air is usually well mixed in the horizontal, all adjacent air parcels will reach saturation at the same height, so the clouds form at the same height. Sometimes air isn't well mixed. You will sometimes see cloud bases are different over water than the nearby land. Lower or higher? Why? Think of other ways, aside from rising, that air could be cooled to its dew point and form clouds.