When they are alone they want to be with others, and when they are with others they want to be alone. After all, human beings are like that.
—Gertrude Stein
Physical solitude, or mental (which can be worse in a crowd) — and its causes, and its consequences.
A total lack of human contact can leave the character isolated even if surrounded by non-humans; and sometimes they have to be specific kinds of humans, and sometimes, no one can keep them from feeling isolated.
Compare
Introversion Tropes. Contrast
Ensembles,
Power Trio and
Duo Tropes.
Tropes
Quite a crowd of them, isn't there?
