Sigmund Freud (1856-1939) wrote a quite bleak work which was published in 1920, called 'Beyond the Pleasure Principle'. In this work he details exactly what the Pleasure Principle is.
It is, alas, not a state of being actively happy or indulging in pleasurable activities. It is, Freud argued, a state where we feel absolutely nothing, not desire, lust, guilt or tension. Just nothing. So pleasure is a place where nothing ever happens!
Freud regarded pleasure as being a state where there is no unpleasure. The psyche cannot cope with any tension and often what we think of as 'pleasurable' activities are actually states of tension. So when we are watching a play or reading a book our minds may be occupied but they are also in anticipation of what is happening and how the story will be resolved. This is a state of tension.
So the only way that we can truly find pleasure is to do and experience nothingness !