It's all a question of perspective

Happiness is a choice.

OK, that statement may annoy some, confuse others and make others want to smack me in the face for not understanding. Believe me, I do understand. I'm not saying that those who are not happy have chosen not to be, simply that if you want to be happy, then you can choose to be. No, it's not that simple, but nothing in life worth having truly is.

What I mean is that you choose, albeit often on a subconscious level, how you are going to respond, emotionally, to the things life throws in your path. For example, you're in the high street, and wanting to go into a shop. You approach the doors, and as you do so a man comes bursting out, knocking into you as he passes, but says nothing, and lets the door fall closed behind him. You can react to this by taking umbrage, by taking the man's actions as a personal attack on you, and reinforce any feelings you may already have of your own lack of value in society.

Or, you can assume that the man has other things on his mind, perhaps he's having a bad day, perhaps he's in a rush to get back to work, or perhaps something bad has happened to him and he's preoccupied with thinking about that, or any number of other things. In this case, you are taking nothing personally from his actions, and are able to just brush them off and continue with your day.
So which is going to bring you happiness? Well, the latter is more likely to, though of course there's no guarantee, it depends how you process the other things happening in your day/life. If you always assume though, that nothing bad is meant by other's actions and words unless you are absolutely certain it is otherwise, then you'll have less in life to be upset by.

And if you have less in life to be upset by, then the chances are you'll be happier.