Because it is natural to us to express more of ourselves. Consciousness is ever expanding and we seek to express more and more of that into our realities.
Of course wanting more does NOT make us tired, it gives us something to strive for. When you were five you wanted a new bike, you got one, then when you were older you wanted a 10-speed, you got one, then a car, you got one. Then a temporary job, that paid minimum wage, then a better job with more money and benefits. The list of things that people want is forever changing, and that is exactly how it should be. How boring would life be if we were stagnant, and never wanted to expand and grow. Once you achieve something I believe it is human nature to then set your sights on the next thing that you want to achieve. After all, once you graduate kindergarten, there is more schooling ahead of you, so that you can learn more than the alphabet, and how to tie you shoes. When I wanted my first house I thought of it as my mansion, it was only 900 square feet, then when I wanted another house, it wasn't tiresome to strive for that, it was simply setting my goals, then achieving my desires.
I think that this is what life is all about, achieving what you want, when you want it.
Because we came into physical to EXPAND. That's why we are here. It's normal to always want more, we are here for that. That's just the essence of the Universe.
If you could change the word "want" to the word "appreciate", you would find the experience to be much more satisfying overall. The word "want" comes from a perspective of lack. When you say the word "want" you are focusing on something that you appreciate from the point of view that you do not have it. If you were to start to appreciate the things that you "want" rather than focus on the lack of what you want in your life, you will greatly improve the way you feel and you will actually begin to love the very things that are making you upset right now. So, if you want a higher paying salary, don't focus on the lack of it right now, because that will only trigger feelings that make you upset. Instead, focus on the better salary from an appreciative point of view. Imagine what it would be like to have that salary and how wonderful and enjoyable life would be if you earned that salary. Find ways to feel good about the salary by imagining buying the things that you would buy if you were earning that salary right now. Do this with all of the things you currently believe that you want. As you begin to view the things that you want in this way, you will feel much better as you think about what you want and they will enter your life much faster, because you will no longer be focusing on the lack of what you want, but you will be focusing on the essence of what you want instead.
Once you start viewing the world in this manner, you will likely highly value your ever increasing desire, because your ever increasing desire expands the world around you and makes your life ever more satisfying as more and more enjoyable experiences and thoughts enter your life.
I believe we don't need many materialistic things to be happy. We want more, because we have bought into the idea, created by society, that materialistic things constitute happiness. And because it doesn't - we keep on wanting more, hoping that one day when we will have everything we desire, we will be happy. Society has sold us a grand illusion and wrapped it with glitter and gold, and made it so appealing that we think we need it. I am not saying that materialistic things are BAD, I am saying that us wanting more and more and more is a very obvious clue that we might be addicted to the very things that we think are making us happy.
Why do you think there are so many poor, starving, dying, unhappy people out there? The rich get richer and the poor get poorer, because we always want more, and in that sense we keep those wasteful capitalists, with their brilliant marketing campaigns and billion dollar assets, going... Yet we are the fuel that keeps the fire going, not them!
We are responsible and we don't like it!
Because being born and living a life in a physical sense would be BORING!
"Go forth and prosper!" ;)