How can pleasure “lack” positive existence? It is indeed the case that our simple common sense seem to attribute positive experience to pleasure and negative experience to pain, that they are the opposite ends of a sliding scale of experience.
However, in order for Schopenhauer to argue that we are doomed by nature to suffering is that pleasure is not positive, but only the relief from something painful.
The reason for this is that pain, suffering that includes all want, privation, need, in fact every wish or desire, is that which is positive and directly felt and experienced. On the other hand, the nature of satisfaction, enjoyment, and happiness consists solely in the removal of a privation, the stilling o a pain; and so these have a negative effect. Therefore, need and desire are the condition of every pleasure or enjoyment. Plato recognized this… Voltaire also says: “There are no true pleasures without true needs.” Thus pain is something positive that automatically makes itself known; satisfaction and pleasures are something negative, the mere elimination of the former. On the Basis of Morality, p. 146
If you reflect in certain terms what all gratifications are, all of them, from a sip of coffee to the deep contemplation of the Last Judgment in the Sistine chapel, you will admit they are either the reduction of the will or its suspension. Now, willing is unquenchable – we may achieve some brief satisfactions, or a momentary relief, but given the nature of existence, they are always temporary and then we go back on the rack. Our normal state of affairs is dissatisfaction. I’m more interested in when we do achieve a sustained satisfaction of our wants, and because of our nature as a restless striving, our chief mode of existence is dissolved and we run up against the inner emptiness that is brought about by the absence of the only mode in which we can exist: boredom. This is all the more true for people who live in an affluent society like the USA.