Buddha's exact meaning of the word may have been lost amid varying historical interpretations, but it includes such connotations as harmony, balance, union, togetherness, calmness, undisturbedness, satisfaction, completion, perfection, and a willingness to accept things as they are. The task of the Buddha is to show people how to transform their existence from Samsara to Nirvana. The frustration entailed in this unattained more is ever-present. Secondly, one cannot always anticipate precisely what will be attained. In the face of such uncertainty, we hope for the better, but to the extent that this is more than we get, we will be frustrated.
Some desire is a natural and normal function of existence.
Without desire for food or water, we would die. The problem seems to be one of excessive desire, or craving. Instead of food or water, I want sumptious gourmet meals and expensive wines. In general, the stronger the craving, the more bitter the frustration. But even here we live in a realm of forever shifting shadows.