Your brain did not evolve to make you happy. Brains evolved for the purpose of improving the organism's chances to stay alive long enough to reproduce. This means that your brain's job is worry, fear, and angst -- the emotions that keep you away from stuff that might kill you. The positive emotions that drive you toward helpful stuff (e.g. food and sex) are less immediately vital to survival and therefore easily overridden by the negative ones.
No wonder we're so miserable most of the time.