The most important reason why happiness is really important because it's extremely essential in the goals that we have in life which will help us achieve those things we wanted to do like our personal goals and ambitions. Being happy and making it as a habit can transform us and also can change other people's lives.
That is how happiness matters. And it is only one decision away.
If you want to bring happiness in life, you have to suffer. Then happiness will come. In order to reach the following goal, you have to work hard first. Only then will you be able to reach your desired goal.