Different people have different definitions for happiness. In my opinion, happiness is really easy, and it mainly depends on your goal.
Some people have very simple goal that is suitable for his current situation.For example, a 35-year-old man only wants a proper salary, a satifactory family with all family members healthy and happly. Then happiness is easy for him. In order to be happy, he only needs to pay some attention to his work and could have enough for his family and life.
However, some people have an ambitious plan,which is far beyond their capability.For example,a poor man wants to become a billiionnaire. Then he has to work a lot of time, sometimes even work extra shifts and hours,and they may have little time for their family and have no time to enjoy life. It's hard for him to achieve the goal and be happy.
Above all, if we have proper expects for ourselves, try to achieve our goal and leave enough time for our life and family, then we could be happy!