Sartre contends that human existence is a conundrum whereby each of us exists, for as long as we live, within an overall condition of nothingness (no thing-ness)—that ultimately allows for free consciousness. But simultaneously, within our (physical world) being, we are constrained to make continuous, conscious choices.
It is this dichotomy that causes anguish, because choice (subjectivity) represents a limit on freedom within an otherwise unbridled range of thoughts. Subsequently, humans seek to flee our anguish through action-oriented constructs such as escapes, visualizations or visions (dreams) designed to lead us toward some meaningful end, such as necessity, destiny, determinism (God), etc.
Thus, in living our lives, we often become unconscious actors—Bourgeois, Feminist, Worker, Party Member, Frenchman, Canadian or American—each doing as we must to fulfill our chosen characters' destinies.
But again, Sartre contends, our conscious choices, leading to often unconscious actions, run counter to our intellectual freedom. Yet we are bound to the conditioned, physical world—in which some form of action is always required. This leads to failed dreams of completion, as Sartre described them, because inevitably we are unable to bridge the void between the purity and spontaneity of thought and all-too constraining action; between the being and the nothingness that inherently coincide in our self.