"The Love Song of J. Alfred Prufrock" by T. S. Eliot is a poem narrated by a cautious, conventional, middle-aged man, the J. Alfred Prufrock of the title. He is quite unlike the romantic young protagonists of conventional love songs, and is aware of the ways he differs from such characters, and the fact that while he longs to love and be loved, the societal scripts for romance really are not applicable to him.
Do I dare
Disturb the universe? (45-46)
In lines 45-46, quoted above, Prufrock is contemplating stepping outside his conventional role and social persona to make a romantic gesture or proposition, but he is worried that such a gesture would be socially inappropriate. He has just described the formal structure of upper-middle class social interactions, as precisely choreographed as a ballroom dance, and is aware that if he steps outside the social choreography, as it were, he will disrupt the framework of his social world.
It is impossible to say just what I mean! (104)
In line 104, Prufrock addresses two issues. The first is the impossibility of ever completely communicating one's interior thoughts. The other is the social "impossibility" of stepping outside the bounds of conventional good manners.
I have heard the mermaids singing, each to each.
I do not think that they will sing to me. (124-125)
In these lines, Prufrock renounces the possibility of a certain type of romantic love. His self-deprecating self-description signals a realization that he is destined by the nature of his character to be an observer to grand passions rather than a participant in them. The mermaids, as the women in his social circle to whom he is attracted, will not sing their love songs to him.