What could explain a never ending desire?

I’ve come to realize that a lot of my unhappiness comes because I seem to always have a desire for more—no matter how much I have. It’s something instinctive. I’ve only recently (this morning?) come to see that it’s there. I think of things I don’t have, but desperately want, and that makes me unhappy.

I’ve been turning my attention to the wonderful things that are already in my life. Thinking of them and appreciating them, and know I can get what I want from them. I don’t have to keep on looking and looking and desiring and desiring.

I suppose other people do the same thing. It might be interesting to hear examples of this. But what I really want to know is why I (or those people who do it) do it. What role does this kind of desire play in our lives? Does it have any positive role at all? Or is it all designed to make us unhappy?