Because due to sin and imperfection we cannot achieve complete happiness, unfortunately, (Romans 5:12) however, God promises us a time when sin and imperfection will be done away with. That will happen in Gods new world. (Revelation 21:4)
Because we are never satisfied. If we get a new car... It looses its joy after a week or two and we need to buy another feel good toy. We are only happy if Christ lives in us AND we help people who are in need. When we help others we become happy.
Do you mean we need other people in our life to be happy all the time? Or why can't we keep ourselves happy all the time? C'mon you know the answer.It's a bittersweet life.No pain no gain. And it really depends on us.On how we think and how we take things.You can either complain and feel like life's unfair blah blah blah or get along with it and be happy.Yes you can be happy all the time.You just have to figure it out how and forget the pain.