Loneliness is portrayed in Nausea as a necessary consequence of being separated from our true selves.

By choosing to live in the past, as part of his historical research, instead of facing up to the present, Roquentin has become estranged from himself. He ekes out a bare existence—it certainly couldn't be called a life—that isolates him from the world in which he lives. So long as he persists in this ongoing act of bad faith, so long as he postpones taking hold of his own boundless freedom to make something of himself, he will continue to remain in this unhappy condition, forever separated both from the people he meets and from what he could be if he only had the courage to make that leap of commitment.

Ironically, it is only when Roquentin realizes that he is on his own in an absurd, meaningless universe—that only he can choose who and what he is—that he is finally able to overcome his sense of existential loneliness for long enough to make a clean break and take the fateful decision to move to Paris, where he intends to write a novel.