Kissing does burn a small number of calories, according to WebMD. During a very passionate kiss, the body can burn up to two calories per minute. For comparison, WebMD notes that running on a treadmill burns an average of 11.2 calories per minute.
Although the number is small, this is double the normal metabolic rate, WebMD reports. Wikipedia mentions that kissing burns calories by exercising the muscles of the face, lips and tongue, and by causing the heart to beat faster due to the release of adrenaline that kissing induces. Wikipedia also notes that kissing has been shown to reduce stress, increase relationship satisfaction and lower cholesterol levels.