Why is "we" important in the line "Till human voices wake us, and we drown"?
The "we" in that line changes the implications of the entire poem. The first lines of the poem speak of a shared experience, and a shared existential dilemma, but after those lines, the "you and I" falls away, and Prufrock's journey around the city, and through life, becomes intensely personal and isolated. It is easy to fall in to thinking that this is someone else's set of problems. Returning to "we" at the end of the poem closes the circle and applies the pains and fears of the poem to everyone. We are all Prufrock, this line reminds us.
Elliot started writing "the love song of J Alfred Prufrock" when he was just 18. What could have been a silly love poem, turns into the philosophical wonderings of moderninsm with Ellliot's use of the plural. Elliot is commenting on the alienation of the individual in society, when if musing to himself, he still uses the plural, to try and make himself feel less alone, less like he is the only one suffering, feeling this way.
We portrays more than one person. In this poem the narrotor first started out with us speaking of himself and the women he was anticipated on telling that he loved her. When the word we is brought about, the narrotor is not only speaking of just 1 or 2 people but everyone.