A related question that could be pondered is it's opposite, or what makes us unhappy, especially in 2024 which factors seem to be increasing, causing increased unhappiness.
 
  There is a significant amount of economic uncertainty and financial instability, especially in the aftermath of the COVID-19 pandemic. Increased homelessness, going further into debt, and fear of not having a sustainable life.

There is a large increase in social isolation and loneliness, which have been exacerbated by the shift towards remote work and social distancing measures. K-Dog has nailed this one down.

There is constant political polarization and increasing civil unrest, that create feelings of anxiety, fear, and division. This makes any "future" for us very suspect.

Then there is environmental concerns, such as the climate crisis and natural disasters, which can lead to feelings of helplessness and despair, because it is causing such major issues with all of life's sustainable aspects. See Maslow's pyramid of human necessities for happiness. Food and shelter being first.

  Which brings us to unhappiness which makes us unhappy, or mental health issues, such as depression and anxiety, which affects anyone regardless of their circumstances.

As resource depletion continues at a rapid pace, the general atmosphere of living for the majority of humanity is very uncertain. The volatility index is very high.