Why are human relationships are impossible to sustain in 1984?
Human relationships are impossible to sustain because of the controlling nature of Big Brother and the Brotherhood. Big Brother requires that everything is subservient to the party, party doctrine, and for the good of the party. Human relationships are individual, personal, private, and very intimate--things that the party wants eliminated and drastically controlled. The party tries to control who marries whom and even the bonds of family are erased, since children are encouraged to "turn in" their parents for any "thought-crimes."
Winston and Julia carry on a clandestine relationship based on their individual lust and longing for each other. They choose to be together, they share their bodies, minds, and honesty. This is contradictory to party doctrine because they are hiding their relationship, they have chosen to disobey the rules and regulations and are "thought and face criminals".
Human relationships are near impossible because everyone is wearing a "mask", enslaved, and afraid of committing "face" or "thought" crime. There's little wonder human relationships suffer.