As Travis goes crazier and crazier (and crazier), he starts watching TV while holding his gun. At one point, he's watching the show American Bandstand, which used to play pop hits while people danced to them. In the middle of the dance floor, we can see a pair of empty shoes, as Jackson Browne's song "Late for the Sky" plays, containing the lyrics: "Such an empty surprise to feel so alone" and "How long have I been drifting alone through the night?"
Both the song and the empty shoes demonstrate Travis' own sense of loneliness and non-being. He's not present in his life, at all—like the pair of empty shoes, he feels like he's in the midst of a world where people like Betsy and Tom are connecting, while he remains alone or invisible. He's not going to participate in the dance of life—instead, he's going to react with anger and vengeance, refusing to join in the dance, and trying to murder his way out of his despair.