Even habit cannot do much in this respect.

A man required to submit to gradually increasing periods of solitary confinement would probably go mad as soon as he had been kept for a year without a break.

A settler, though he may be the son of a settler, and may have known no other way of living, can hardly endure existence unless his daily intercourse with his family is supplemented by a weekly chat with a neighbour or a stranger; and he will go long and dangerous journeys in order once a year to enjoy the noise and bustle of a crowd.
But, on the other hand, the nervous system of most men will not tolerate the frequent repetition of that adjustment of the mind and sympathies to new acquaintanceship, a certain amount of which is so refreshing and so necessary.

One can therefore watch in great modern cities men half consciously striving to preserve the same proportion between privacy and intercourse which prevailed among their ancestors in the woods, and one can watch also the constant appearance of proposals or experiments which altogether ignore the primary facts of human nature in this respect.

The habitual intellectualism of the writers of political Utopias prevents them from seeing any 'reason' why men should not find happiness as well as economy in a sort of huge extension of family life.