We are guided by a set of internal as well as external variables.There are cases when internal ones are responsible and in some cases external or internal and external both.
Yes human wants are unlimited and we dont satisfy with what we have which makes people think about future and present situation...due to worry happiness is never attainable.
It is not a question that why you feel that life is too short, The question is that how to remove those factors which gives the feeling that life is too short
If one is the very source of unhappiness he or she must look inward and if the sources lie outside one can control which are controllable otherwise it's best to leave them at that!