Unhappiness, like any other emotion, is only a result of external circumstances if one lets that be the case. Yes, some (even many) things can happen that are beyond your control. But, the way you respond to these things is within your control. And, ultimately, your response is the final result to all circumstances, both exteranl and internal. Like it or not, we are all responsible for our own happiness. Some people just choose to be unhappy (or negative or any other emotion you can name).
No one would deny that bad things happen. No one would even deny that bad things happen to good people. That isn't really the point. Yes, some external circumstances can set someone up for negative thoughts. And, only a fool or eternal optimist (same thing?) would say that they never get unhappy. But, like almost anything else, there are degrees of reaction and there are degrees of unhappiness. In the end, your reaction is the ultimate barometer.
Let's take an example. You're going to a party for a friend's birthday. You've left the house plenty early and shouldn't have a problem arriving on time. But, wait. What's that up ahead. Of course it's road construction. No problem, just take an alternate route. Turn the corner, go a few blocks and there's an accident up ahead. Another slow down. Every move you make is countered by the evil traffic gods with a diabolical twist. You do everything within your control to get to this friend's house on time. External circumstances have thwarted your plans for being on time. You should have been there in a timely fashion, but due to circumstances beyond your control, you weren't. Anyone would agree that it isn't your fault. But, how do you react?
This is what separates the mentally healthy from the chronically unhappy. Admittedly, if this were my story, I would initially be very angry. I think anyone would. However, you are going to a friend's house on his/her day. Do you want to walk into the party in a negative mood and drag everyone else down? Or, get angry, get over it and move on? Those are the choices, really. You can choose to stay in a down mood- to stay unhappy. Or, you can remember why you're here in the first place and try to be happy for your friend. I'm not so naive or Pollyanna-like as to think you can turn on the happiness with a switch. But, is it too much to ask that you arrive at the party, make your excuses for being late, then smile at your friend for having made it another year? I don't think it is, but I've known a lot of people who like to wallow in the unhappiness of the situation. And, that's a part of the situation that is within your control.
When considering unhappiness as the result of external circumstances, I believe that it all comes in degrees. A person can blame outside forces for his own unhappiness and, to a certain extent, be correct. But, it is the way a person reacts that will determine his ultimate happiness-or lack thereof.