Happiness, for lack of a better word, is boring.
Happiness is boring in the same way being angry al the time is boring; people evolved to notice change. Every single one of our senses only works by detecting change.
Take smell for example. Your nose detects a difference in the smell of the air. When I burn things in the oven, I get used to the smell after a while but when my wife comes home she says: 'Ahh, cooking again earlier?'
Taste is the most obvious one. Fast for a month then eat a corn chip. You will taste that simple food so deeply you could pinpoint the field the corn was grown in. Now eat a million corn chips. You quite literally won't even taste the last one.
Put a fan on in your office, you'll forget its on. Randomly have someone turn it off and the difference will be a bit painful that change is so obvious.
Gain 100 pounds, you get used to the feeling of it. Lose the 100 pounds, you'll get used to that feeling too. If you didn't, people who were obese would strive to be out of pain. There is pain in being heavy, but you get used to the feeling of your body.
I have heard a version of "Humans adapt" for my whole life. I think it's more than that. I think it's more like 'Humans ride the wave of change." We are so symbiotic with change we don't even notice that change is the thing we both fight and enjoy.
Why do I rewatch episodes of MASH? Because it's both the same and different each time. I watch all eleven seasons over and over and what happens? I notice nuance. That's me detecting change. It's hard to exhaust the nuance because there is just so much of it. I see patterns, like how Hawkeye acts drunk or how Hot Lips manages her hair.
What is hard is FORCING change. That's the tricky bit. Lose the weight, win the lotto, date your dream girl. Now that evolution that has kept you safe by detecting change fights you because you are way outside your comfort zone and what you are used to.
Subconsciously, many of us, would do the things that will help us get back to safe harbor: stifle and defend against self imposed change.
I think focusing on happiness is a poorly defined goal. I'd say to enjoy happiness but strive to have peace.