Everyone wants to get fully satisfaction but mostly it is impossible and our desires have no end. I strongly agree with people are not satisfied with what they have. They always want something more or something different .I think that there is two possible factors : human nature and environment.
First reason is related to human nature. It is about human characteristic to be jealous and bore quickly something they used before. We are jealous and it is related to hope to be rather than others. For example, to be same with someone is not good idea in most people as well as they want to be different and better. Besides, we always look for new things that it is better than before.
Another reason is our environment and social. We follow our close people's choose, and their advice always influence our urge to wish new or different thing. For example, after my friend buy new car, we start to think about purchase this one. Furthermore, people tend to pursue social tendency.
Finally, is likely to be perfect, and this behavior that we never satisfied what they have is secret of rapidly developing world. On the other hand, if you cannot achieve your goal and buy wanted thing, it is to be torment's source.