—Yet when we came back, late, from the Hyacinth garden, Yours arms full, and your hair wet, I could not Speak, and my eyes failed, I was neither Living nor dead, and I knew nothing Looking into the heart of light, the silence. (37-41)
A female speaker recalls a time when someone sent her hyacinth flowers, which seems like it was a pretty great time in her life. But at one moment, for reasons she doesn't seem to understand, she lost her sense of excitement, and just went numb. She completely lost her ability to communicate and felt like a zombie, "neither / Living nor dead" (39-40). This scene could be a symbol for the fall of modern minds, which were once connected to each other with a sense of love, but then lost their ability to communicate or think straight, and this led them spiraling into an isolated silence. "The Waste Land" as a whole could be seen as Eliot's attempt to make sense of where this isolation has historically come from.
Sighs, short and infrequent, were exhaled, And each man fixed his eyes before his feet. (64-65)
The speaker describes a crowd of people flowing over London Bridge like zombies. None of them seem to pay attention to each other, since "each man fixe[s] his eyes before his feet" (65). This image of staring at the ground in front of one's feet is a perfect symbol for how our daily routines tend to dull our attention until we no longer even realize that we're just moving in a giant herd of people (ever texted in a crowd? Same deal). For Eliot, there is a certain emotional deadness or numbness that has made people stop paying attention to one another, and this is something society will have to overcome if it's ever going to get out of the waste land.
So rudely forced; yet there the nightingale Filled all the desert with inviolable voice And still she cried, and still the world pursues, "Jug Jug" to dirty ears. (100-104)
After mentioning the myth of Philomela, the speaker implies that modern people are like Philomela, insofar as they've had their "tongues cut out" by the collapse of meaningful communication in the modern era. Like Philomela, these people might try very hard to express themselves in a beautiful way, but all other people can hear with their "dirty ears" is nonsense. Everything meaningful just sounds the same, like "Jug Jug." This inability to properly communicate is a main cause for the loneliness of modernity.
"My nerves are bad tonight. Yes, bad. Stay with me. Speak to me. Why do you never speak. Speak." (111-114)
An unknown voice suddenly butts into the poem, filling the page with really anxious, neurotic demands. The person asks an unknown listener (which might be you) to spend the night with them. Obviously, this person doesn't want to be alone at all, but at the same time, they want to completely support the conversation. The anxiety outlined in this passage belongs to a person who needs to have a conversation, but has absolutely nothing to say (sound familiar?). This person also seems to be made very paranoid by silence, so they ask you to say anything at all to make the silence go away. We are dealing here with the kind of nervous, super-needy person that the modern world produces, and Eliot suggests that this type of person is created by the collapse of people's ability to properly communicate with one another.
[…] The nymphs are departed. And their friends, the loitering heirs of city directors; Departed, have left no addresses. (179-181)
At one point, it might have been possible to sit alone on the bank of a river and feel like you were communicating or communing with nature. But unfortunately, modernization has completely destroyed the magic of past generations. People used to live in a world filled with poetry, where fireflies hovering over water were believed to be supernatural nymphs. But nowadays, all you'll find next to a river is a bunch of sandwich papers and beer cans. A rationalist would tell you we've come a long way since the age of superstition, but Eliot's not so sure. He feels there's something super lonely about a world without poetry.
We think of the key, each in his prison Thinking of the key, each confirms a prison (414-416)
In these lines, Eliot gives his most powerful image of modern isolation, comparing individual existence to living inside a locked prison. This suggests that people might actually want to stop being so self-absorbed, but they don't have the spiritual guidance or mental strength to do so. We might often think of the key that will release us from the prison of our isolation, but that doesn't mean we have the key. This image comes up fairly late in the poem, and there's no time for Eliot to dance around the issue anymore: if you want to stop feeling isolated, then you've got to start thinking of others just as much as yourself, or even more. There's just no way around it; you think about yourself all the time, you're going to feel lonely. That's just spiritual arithmetic, as far as Eliot's concerned.