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.
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
. See The Singles Index
, which deals with romantic solitude.
Quite a crowd of them, isn't there?