The key to ultimate happiness is within your own thoughts. Negativity is always around, knocking at your door trying to pull you down. If you allow it, of course it will guide you on a course of destructiveness and unhappiness. Each individual is responsible for self once reaching a certain plateau in life. It's been said time and time again that thoughts manifest themselves into real things, believe it. Doesn't have to be anything major either.
Start your day with meditation, have conversations with yourself....it's perfectly normal. For at least 30 minutes at the start of your day, take time to tell yourself that it will be great and productive; go over short term goals for the day, motivate YOU. Even when things happen unexpected that change your mood, don't allow it to take over and consume all thinking or the remainder of your day, instead rather, recognize it, face it and carry on.
How many of times has one small not so good thing, e.g., running out of gas on an extremely hot day or a driver cutting you off in traffic completely turned the you upside down? Next time, acknowledge the inconvenience it may have brought but don't harbor on it. That small, little corner of negative thinking inside your mind can turn into an u controllable wildfire. Positive thinking spreads the same only, for some reason we clearly see a little piece of ugly before we can even recognize a mass of goodness.
“The road to success and the road to failure are almost exactly the same.”
-Carl R. Davis
As are the roads of happiness and pain.