It first struck me many years back while in a transport on a road in one of the many cities that witnessed my childhood. I happened to see a young boy of inconsiderable means - a street urchin as the sort is referred to. He was laughing, his head thrown back in mirth, possibly over some comment of his begging companion or on witnessing something on the road. In that one moment, his means, the dirt, the hunger, the bleak future - they were all forgotten, sacrificied in favour of the mirth he had dipped his mind into. I wondered how convenient it would be if one could freeze time to that one moment - the boy then would be in happiness forever - no adversity would face him. We are not lacking in moments of joy - whatever our circumstances or station in life - our problem is that they are only temporal.
Some months back, as I was with some amusement and some pain, reflecting on my own state of mind, which had from relative happiness, plunged to some pain and with the passage of time and some more events, jumped to an even happier frame than before it sank. It seemed that any feeling of joy or sorrow seemed particularly intense (if not exclusive) to only those times when a change in circumstance happened. That it to say, there seemed to be no inherent elation or despair in any particular state (or set of conditions) - these feelings manifested themselves only when there was a change. So a change for a poorer set of conditions would generate sorrow and for the better - joy.
If I were to illustrate with an aid of a very simplified example - if you were on an income of Rs 100,000 a month and I were on Rs. 5000, and if both of us had been at our respective levels for some time, we would both be (all other things being equal) equally happy / unhappy. If however, I found my income the next day increased to Rs 100,000 my joy would be intense, equalled by the depth of your despair should you find your income fall to Rs. 5000. There is a side road here on the relation between hope / expectation and happiness, but I will go down that some other time.
So, the point being - happiness is a function of state change and not of any state in itself. Actually, to be more general, there are two sources of happiness - that of a state itself (Hs) and that of state change (Hc) where Hc seems many times higher than Hs.
Hc however, fades with time - it is felt keenly in the immediate aftermath of the state change, and fades with time as one becomes accustomed to the new state - and memories of the old fade.
Therefore, H = Hs + Hc(t)
With Hs negligibly small as compared to Hc(t)
so, H ~= Hc(t)
Which makes me look like one of those management gurus who illustate their points with equations or pyramids or concentric circles or if they be more accomplished - with equations below pyramids surrounded by concentric circles.
Anyway, taken in conjunction with the first case, we see that to have H as high as possible for as long as possible (which seems to be in some form or another, a common goal of man), our state itself does not matter if we were to rest on it - what gives happiness is only rapidly changing each state for the better, without resting much on our laurels. Meaning, money or fame for example, are of no use but more money and more fame are to be welcomed. Which also leads to a painful conclusion - no matter how much one gathers or achieves, one can hold onto happiness only by gathering or achieving more. If you can see a Tantalus somewhere - yes, continually longing for more (and getting it) is the only known way to happiness.
Now, for a spanner in the works. We have referred so far to happiness. Change the signs and we're talking sorrow. So,
S ~= Sc(t)
with an understanding that there is an Ss, but which is inconsiderable as compared to Sc(t) (remember the boy).
Now Sc(t) is a sorrow caused by the worsening of conditions. It too, thankfully, fades with time and memory, but always lurks around as long as there is a set of conditions worse than the current. Which means that the more one gathers or achieves, the more one is at the risk of Sc(t). So the most impoverished, fate slapped survivor is at little risk of an Sc(t). He has to contend only with Ss.
So, you see that your goal of eternal happiness can occur only by freezing time, or by rapidly changing states for the better. However, with each ascension of state, you risk losing it and landing up with Sc(t).
Damned if you do, damned if you don't.