Over the history of man, philosophers have debated the meaning of life and attempted to define happiness. I posit that the key to happiness is in biological fulfillment. We have a gift of a mind that leads us to believe we have free will. To some extent our will is free. But to a large extent, and for practical purposes, it is not. We live to fulfill our biological needs. It sounds obvious, because it is. Without fulfilling our biological needs, we can not survive, so it makes sense that our brains and bodies are hard wired to constantly be seeking that biological satisfaction. What is that biological satisfaction? We can start with the basics. We need food. Food and water. Oxygen. A human can survive 3 minutes without air, 3 days without water and 3 weeks without food. Regardless of conscious choice, all humans will seek out food, water, and oxygen. Shelter and clothing help us to survive. Sex is a biological need. Social and family companionship are biological needs. Good health is a biological need. Bad health leads to an early death.
Happiness is linked to the degree to which these biological needs are satisfied. If you have to go days without food or water, then you are less happy than if you eat three square meals a day. If your three meals a day are bread and water, you are less happy than if your meals consist of a variety of choices. If you breath dirty air, you are less happy than if you breath fresh air. If you live in a shack, you are less happy than if you live in a mansion. You feel safer and more comfortable in a mansion and more likely to survive the elements.
The purposes we seek out in life are really just means to acquire and satisfy those biological needs. Satisfying those biological needs brings us happiness. If one becomes a doctor, one is likely to earn more money, thus affording more meals, nicer shelter, and a more comfortable biologically satisfactory life. Having access to more meals, nicer shelter, and a comfortable life also attracts a sexual mate who desires those things as well. Thus, achieving the successful career also assists in satisfying the sexual biological need.
Humans come in a wide variety of abilities. These abilities are our biological nature. Our DNA. Some humans are naturally more intelligent than others. Some humans are naturally more physically strong and coordinated than others. Some humans are great at science while others are better at cooking. Some humans have higher IQs than others. Achieving happiness then, is also tied to discovering what our natural biological gifts are and utilizing them in the best way to achieve our other biological needs. If you ask a fish to climb a tree, that fish will not be happy because it is not biologically capable. It will constantly struggle to achieve that goal, making it less happy than if it were tasked to do what it is biologically good at, swimming.