Dogs pant and let their tongues hang out of their mouth to regulate their body temperature, because they have fewer sweat glands than humans. However, several other explanations exist as to why dogs hang out their tongues.
Some dogs with functional tongues have a mouth cavity that is too small. This may be caused by genetics, an abnormal jaw or missing teeth. Dogs who are chronically unable to pull the tongue into the mouth have hanging tongue syndrome. The tongue dries out due to constant air exposure and can become cracked and painful. Pet owners can alleviate the discomfort by applying olive oil to the tongue as a lubricant, encouraging frequent drinking and allowing the dog to lick ice cubes.