The main issue on Prufrock's mind is a deeply personal and intimate one--he wants to ask someone that he cares about an "overwhelming question" of serious import. That subject--whether to ask her, when, how, and what impact it might have, and if he will be rejected-- is the subject that his mind keeps coming back to. It is a personal, intimate subject, and the reader can feel that closeness. In the meantime, however, his mind wanders. In the stream-of-consciousness style that Eliot uses, he lets Prufrock's mind wander about as he ponders this issue. It reminds me of someone sitting on a porch or a couch, thinking about what to do, but it's stressful, so they kind-of avoid focusing too hard on the subject, and let their mind stray and wander about to avoid thinking about it too much.
So, to specifically answer your question, look at how the poem starts--it begins with a very distant perspective, inviting the reader, or the person to whom he's speaking, to go for a little walk through the run-down part of town. "Let us go then, you and I," he invites, and the invitation is distant and impersonal as he describes the journey through the "sawdust restaurants." He then uses a transition to bring it back to the personal; he describes the streets that wind around town, which remind him of a "tedious argument of insidious intent." This leads him right into the personal--he brings up "the question." The streets are like an argument; this is stressful, and reminds him of asking the question. That is personal. But he very quickly withdraws from that and gets impersonal again by saying, "Let us go and make our visit." He's avoiding the personal again, and distracting the reader.
He proceeds to describe the fog, parties, inane conversations, etc. But each time, something in the ramblings of his mind reminds him of the personal, and he comes back to it. It is in this way that he transitions from the impersonal to the intimate. I hope that those thoughts helped; good luck!