A few observations. I consider myself smart, but far from smartest.
tldr: intelligence lets you control some variables for happiness, but it is not the entire happiness function.
1. We're social and emotional beings beings. Emotions are more powerful than intelligence. We gladly choose something that we enjoy over something that will make us happier long term. All humans do this, mental fortitude is in short supply.
2. Life, and society move the definition of happiness constantly. Either through manipulation, new discoveries, new laws, new constraints. So even smart people need to adapt. If yo were happy last year, maybe today you have to pay higher taxes, or your country gets into a war, or you get a new president, or you're just competing with other smart people in an endless game of chess.
3. We move our own goalposts. Think the rich who thought wealth was the source of happiness, then they get there are are miserable. Or simply spend their time looking for new challenges, infinite growth, etc.
4. Living in a society means your happiness is always, always dependent on the state of that society, and therefore the happiness of others. It's like fluids. You can't be the one drop of water in a bottle that is red. It will dilute, and vice-versa. Maintaining a state of happiness in an unhappy society means building barriers and constantly fighting "them".
5. I have a theory that live is like a wave of ups and downs. The sum of all your highs will equal the sum of your your lows by the time you die. You have some control of when, how much and for how long to be happy/unhappy (unhappiness also means hardship, work, selling time for money, etc, the things we use to pay for happiness), and that controls how the quotas get distributed throughout your life. I have absolutely not tested this, I am still experimenting with my life. Counter examples may exist, I have a sample size of 1.