A little learning is a dangerous thing ;
Drink deep, or taste not the Pierian spring :
There shallow draughts intoxicate the brain,
And drinking largely sobers us again.
Fired at first sight with what the Muse imparts,
In fearless youth we tempt the heights of Arts ;
While from the bounded level of our mind
Short views we take, nor see the lengths behind,
But, more advanced, behold with strange surprise
New distant scenes of endless science rise !
So pleased at first the towering Alps we try,
Mount o’er the vales, and seem to tread the sky ;
The eternal snows appear already past,
And the first clouds and mountains seem the last ;
But those attained, we tremble to survey
The growing labours of the lengthened way ;
The increasing prospect tires our wandering eyes,
Hills peep o’er hills, and Alps on Alps arise !

~ Alexander Pope

There is no state of affairs or situation that remains permanent and the wish for nothing to change is often the cause of sadness and frustration for many people.

Another thought is of course that happiness and sorrow are only programs in our brains, put there by evolution to make us do the things required of us to survive and reproduce. And they are in balance. I pick berries because they have food value, and nature rewards me with a nice taste to make me do it. As the berries get fewer and I have to reach further the thorns prick me and the pain tells me to stop, because the damage to my body now outweighs the food value of the berries. The stick and the carrot if you like, and as you must know if the donkey escapes from fear of the stick, or gets the carrot, it stops running. And of course nature needs you to keep running.

Freud/Lacan posit human drives as the source of our happiness and sadness, the death drive being a major drive/force in humans.

I think something along these lines is more successful than trying to pin it to some conception of reality. I tend to agree with Gilles Deleuze that the only reality we can know are the shadows that play on the walls in Plato's cave.

I prefer reality, but I do remember reading many years ago, (so don't ask where to find it ) that a medical study performed by a group in Oxford, found that, people suffering from clinical depression were much better at predicting the future than the average person. They asked a series of verifiable questions, like do you think shares will rise, will the war be over by, etc. and then checked them against what really happened. Which means that most people are deluded and the delusion is usually far too optimistic. Be happy.

I've heard about studies showing people who lie less and are aware of reality are more depressed.

I prefer reality over fantasy.... You know what you're getting without any doubts or uncertainty...