Why does seeing his half-sister and his father reveal to the narrator "the desolate loneliness" of his position?

The narrator spends many evenings at the Opera, and one night, he notices a striking young girl in the audience. She is with her parents, and when the narrator looks a bit more closely, he notices that man is his own father, meaning that the girl is his sister. He leaves the building, feeling suffocated and miserable. He cannot decide whether he wants to weep or curse. The narrator feels abandoned with no identity.