If Prufrock is meant to represent modern existance, than modern existance is alienating and lonely. The speaker cannot relate to people, and uses the word "us" rather than "I" because he is so lonely. He thinks constantly about what others would think about his actions, but he has no idea as his social interaction with people is so limited. He is addressing himself the way most people address a friend. THe last half of the 19th century was seen as a time of less human interaction, a time of more industry and capital, it is most likely this that Eliot is evoking with Prufrock's alienated and lonely speech.