It is a well known proverb that "Humans are social animals and they can't live without each other".
.... yes of course.... great start!
Well one may think that being biased , egotist, cynic and having misrepresented personality isolated one person from one another.
.... How about introverts? They love isolation and do not like to mingle with others...So, for some people, it's in their DNA ...lol
Firstly if subject 'A' had a bad experience in his/her life and he starts living isolate d and if subject 'B' wants attachment so badly with 'A' and fails to do so.
... why do you say "subject A" ? I feel you should treat him or her more politely ...LOL .... I prefer the word " person A" to "subject A". :D
Thirdly being biased also isolates people, for example in one family if parents love some children more than others