The isolation starts early and sticks around. The poem's second line places the speaker alone in his room in the wee hours in the "soundless dark." This description of the dark as "soundless" is important. There isn't a taxicab horn, a dog barking in the distance, the sound of crickets—nothin'. "Soundless" makes the dark and the isolation feel more intense.
That this is what we fear--no sight, no sound, No touch or taste or smell, nothing to think with, Nothing to love or link with. (27-29)
Larkin ups the isolation level in these lines. Before, it was just the soundless dark. That was pretty bad. But now, as the speaker contemplates death, we have all the senses muted. It's in these lines that Larkin addresses the speaker's sense of isolation most directly: "nothing to love or link with." It's this connection that our speaker longs for in life and that will be impossible in death. It's the loss even the possibility of this connection that he dreads most.
And realisation of it rages out In furnace-fear when we are caught without People or drink. (35-37)
When is the realization of approaching death the worst? When is it the hardest to deal with? We'll tell you when: it's when no one is around. Contact with other people (and, apparently, booze) helps to keep the fear of death at bay. Our speaker doesn't seem to have many friends, so that would explain his habit of getting "half-drunk at night." This guy really needs a Facebook account.
Postmen like doctors go from house to house. (50)
The poem's last line is linked to the theme of isolation. When we are isolated, cut off from human contact, we feel closest to death: isolation is the enemy, connection is the remedy. We know what you're thinking: "That makes sense, Shmoop, but what's that got to do with postmen and doctors?" Well, you're in luck because Shmoop is going to tell you. Actually, we're not. We bet you can figure it out for yourself.
Just think about it. In a basic sense, what do doctors try to do? They try to keep us healthy. They try to keep us alive. What do postmen do? They get chased by dogs, sure, but that's not the point. They deliver mail. They deliver letters—letters from other people, letters that make us feel loved and connected. See the connection? We bet you do.