Maybe sometimes happiness has a reason. But I think most of the time you simply find yourself with happiness thrust upon you, while you seek to rationalise to yourself the cause of this emotion. This has its uses, I'm sure; if you can identify correlations that may help you do things to increase your happiness. But I believe it is just as important to just indulge in the feeling. To be thankful for the inexplicable delights coming your way is a good exercise in finding happiness in whatever situation life throws you into. A lot of the time reasons are beyond our comprehension, anyway. We're not only subjugate to the chaos of this world, we are chaotic systems to our very nature.
Not that I think that maximising happiness is or should be the supreme goal of life. Emotional constancy would nullify the whole concept of happiness, and besides there are other more important and interesting values to strive towards. But it is nicer to be happy than to be sad, and so there is no shame in enjoying the heck out of it while it lasts. I found this music today. It fills me with happiness. I have no idea why, and I'm content to leave it at that.
Love and happiness,