Why do we value happyness and why should we try to be happy?

Why should we want to be happy? I suffer from depression and am on medication and I wonder sometimes why everyone says people want to be happy? What is the motivation for it? We are going to die anyway. And why do we feel that happyness is somehow better than sadness? Why should we try to make the best of this life, and who decides that the way to do that is to find happyness? The point is I see no reason to muster the energy needed to be happy. This is a very philosophical conundrum for which I really need to find the answer. Please help!