What is Bradbury trying to say about Montag’s state of mind in the "one drop of rain" paragraph on page 15? How is Montag changing?

This paragraph is meant to show how quickly Montag is changing his view of the world and how, in this moment, he is overwhelmed by that change.
He has just met the lively and curious Clarisse, who made him question whether or not he's happy, and come to the conclusion that he "wore his happiness like a mask" and is not truly happy with his life. Right after this encounter, he came home to find his wife, Mildred, nearly dead from an overdose and had to call in emergency services to save her life.
The "one drop of rain" paragraph comes after Mildred has been saved by some callous, uncaring medical machine operators. With Mildred now asleep, Montag thinks about how only an hour ago, he had been on the street with Clarisse and that it was "Only an hour, but the world had melted down and sprung up in a new and colorless form." When he opens the window, he can hear Clarisse and her family talking in their house, and he wishes he could join them. The one snippet of conversation that he catches is Clarisse's uncle talking about how people in their society use each other and move through the world without knowing how to care about others.
Montag's "one drop of rain" sequence shows us that he is taking in all the information of the past hour and searching frantically for answers to try to make sense of what's going on in his life. This is a turning point for Montag—he knows that he is not content with the life he has, mindlessly burning books, and that something needs to change. His statement after the end of the paragraph, "I don't know anything any more," shows that he is questioning everything now, rather than blindly following, and is starting his journey towards individualism.