I think uncontentment is one of the reasons why people are unhappy. I think it is human nature to expect more and want more than the things that they have. For us to be contented, we should appreciate the things the we have. Our family, friends, job , loved ones are the reasons to be thankful for. Depression is also a common reason of being unhappy. Being a positive person and having an active lifestyle would be a big help. We should always look at the brighter side of life and always think of happy thoughts.
I agree, when a person is not contended with what he has, it brings about unhappiness in him because he would try to find ways to do something that can try to alleviate it. Another reason is non-fulfillment of one's wants and dreams. It creates a void in a person that he continuously wants to fill, and if he doesn't, it could be another source of stress and unhappiness.