Bernard is lonely because he doesn't fit in and has a "reputation."
One of the ways that the author uses language to convey Bernard’s loneliness is in using other characters to describe him.
Bernard Marx has an “unsavoury reputation,” so he is lonely. For one thing, he does not spend as much time with the girls as other men. He is not as promiscuous. Other men have many, many girls. That is what society finds acceptable. Listen to the description between Fanny and Lenina about his reputation.
“But his reputation?”
“What do I care about his reputation?”
“They say he doesn’t like Obstacle Golf.”
“They say, they say,” mocked Lenina.
“And then he spends most of his time by himself-alone.” There was horror in Fanny’s voice. (Ch. 3)
Lenina describes him as “absurdly shy,” when she thinks of him. This makes it clear that Bernard does not spend much time with people, girls or otherwise. He asks Lenina to go to the reservation with him, but she only agrees because she will not be alone with him much. He is considered weird.
The author also point out Bernard’s physical inadequacy, which means that he does not fit in with his caste.
For whatever the … Bernard’s physique was hardly better than that of the average Gamma. Contact with members of the lower castes always reminded him painfully of this physical inadequacy. (Ch. 4)
This makes him insecure because he feels that the Gamma’s are not going to respect him. The author makes this very clear, and uses language like “his self-consciousness was acute and stressing” (Ch. 4). Bernard never catches a break. His size likely contributed to his personality, or perhaps it is just part of who he is. One way or another, the total package makes him miserable.
The author also uses the word “apartness” to describe Bernard.
Too little bone and brawn had isolated Bernard from his fellow men, and the sense of this apartness, being, by all the current standards, a mental excess, became in its turn a cause of wider separation. (Ch. 4)
The words all create a sense of being lonely. We have the word “isolated” and the “sense of apartness” and then “separation.” He does not fit into his culture. He is the other, right there among his fellow man. He does not take whatever woman comes along. He does not play the games, or take the drugs. He does not seem to belong anywhere. In their society, this is not acceptable. Every little way in which he is different just pushes him away.
This is a dystopia about a world where everyone is happy, told from the perspective of one man who is not. From Bernard we see an outsider, and therefore we do not see an easy, carefree world. As much as they have tried to eliminate all of the suffering and make everything about joy, it is clear that they have actually done nothing of the sort in their push for consumerism and hedonism. All they have done is create a world of emptiness, where everyone is alone.