He had noticed that all individuals, without exception, tend to seek pleasure, from the egotist whose attitude is evident, as he uses others to satisfy his own desires, to the altruist who finds his own satisfaction in renouncing his impulses for the sake of a loved one or in the name of an abstract principle, such as honor and duty.
From a psychological perspective, it ultimately seemed that both extreme cases - the egotist who uses his peers and the altruist who sacrifices himself for them - had the same goal: their pleasure, albeit in entirely different ways. We also encounter something similar in the purely sexual realm, with the sadist who mistreats and humiliates the beloved object, and the masochist who seeks to impose this treatment on himself: two opposing manifestations of the same quest for pleasure by the individual.
From certain clinical observations, Freud came to discern that the majority of mental phenomena are subject to the pleasure principle.