We Always Want What We Cant Get

In all situations we as humans always want what we cant have. For example: Doesn't it seem like whenever someone is trying to eat healthy, they always crave those deep fried chicken wings drenched in fattening sauce or that bag of calorie packed cheetos? It also happens in other situations. It is impossible for all of our wants to be met, there is not an unlimited amount of resources and money, therefore, its impossible. Though not always getting what you want is difficult and frustrating, I believe it is also a good thing. We work really hard for what we want. If we just got everything handed to us, we would never be ambitious. Relationships are another good example of how we always want what is not available to us. I for one know that when I was involved in an almost 3 year relationship I never thought it would end so I didn't appreciate it as much as I should have. Then of course when it ended in October, I finally realized that I took it for granted. Friends are desired most when they are unavailable as well. I know I will always have my friends but sometimes I can be snotty with them when I shouldn't be and it makes me think about where I would be without them.
To sum it up, if we know that the things we want are the things we desire the most, why do we take them for granted when we have them and why do we always want more? I believe that it is all just one big competition in life to have the nicest things and the highest status.