Every human being experiences elevated mood due to different stages of life. Feeling or experiencing good or low mood is normal. Such feelings is basically needed to stay lively and to being responsive to our surroundings. Otherwise people will be totally predictable and robotic. So experiencing changing mood is necessary to add variation in life. But there is a difference between depression and elevated mood. The difference could be described as the difference between normal experience and illness. Because every people does not develop depression due to ups and downs in life.So what is depression?