Happily unhappy?

I had the most peculiar of days today; I was happy, frightfully happy! Most unusual because it’s almost exactly two years to the last time I was happy; don't ask - a succession of unfortunate events! By happy I mean not euphoric or anything extreme but cheerful, sanguine. As a child, happiness was synonymous with exuberance – yes, I was an unusually happy child – my expectations now are more realistic (realism, how I detest that word; so often does it mean disillusionment and negative adjustment of hopes and expectations). But I digress.
One would think that I revelled in the unexpected clearing of the clouds of gloom but they would be off by quite a margin. Because as it happened, I spent the entire day fraught, oppressed by the fear that my joy would vanish as it had come, unexpectedly; the dread was more than equal to the happiness. I was also anxious to know what had occasioned my improved spirits; things don’t just happen, do they? Doesn’t recovery from despondency require some endeavour from the afflicted? It may appear that am looking a gift horse in the mouth but am not. I’d simply like to understand the nature of happiness so that I can make it replicable; I’d hate to be at the mercy of fate! I did nothing unusual that could have occasioned the change. I have a dreadful start to my day, made worse by unending traffic jams; took a chance – which I won’t speak of unless it materialises, read an email from my girl. And voila I was happy. Then I reread an email, which I received three months ago, from a most lovely man but which I had discounted as inane. But in my newly found cheer I found it touching, so I replied most thoughtfully.
Having experienced a day of happiness, and two years of unhappiness, I hereby give my two cents for the latter: when at rock bottom, there’s nowhere to go but up, which rules out anxiety, the forecat is always bleak; despondency is explicable, and therefore more predictable! And one's less likely to be deluded by sweet nothings when depressed.