Are these two different things? According to me they are two same things:). They are feelings. Although, one cannot deny the difference in these two feelings, they are in one sense same.
Happiness is for a moment. The moment of happiness can span from a microsecond to years to lifetime. And after this momentary happiness, do you find sorrow always? No! There is a third thing in between the two. Life is not just happiness and sorrows.
The third thing is the state of your mind and heart. When you are not happy and when you are not sad, what is that state of mind? And if you observe, you are in this state for the maximum amount of time, where you plan for more happiness, and where you meet unexpected sorrows...
Happiness and Sorrow are also state of mind. Then these three things are no different. The difference is in the way you feel when you are in that state. Dont you feel, if you could control your feelings, then there would be only happiness? or only Sorrows? or nothing at all? :)