Your brain has evolved this way to keep you alive. So when you are unhappy, your mind is trying to tell you that something is wrong. In fact, a perfectly working brain will make sure you identify all problems, potential worries and potential dangers.... And it engages fear to make sure you avoid them.
Let us be quite clear, I am not talking about clinical depression or any other physiological problem. I am talking about day-to-day stress and strain.... Yet if you do not deal with the day to day misery, it soon becomes a habit that is hard to break.
So how can we deal with this fear-based reasoning? One of the best ways is to live with passion and purpose, rather than fight-or-flight. Become more physical - deep breathing, exercise, go for a run.... Be physically positive. Learn Tai Chi. Meditate.
Or as I say... do good, and you'll feel good.