Wow, Charlie's feeling pretty fatalistic at the beginning of this letter: "It can't be like it was. And I wasn't ready to say good-bye just yet" (4.1.2).
He's acting like he expected to lose everything from the beginning.
For the last week or so, he's been doing a lot of people watching—at school, at the mall. Sometimes he follows people around to see how they act. His behavior toes that fine line between really interesting in a psychological way and really creepy in a weirdo stalker way.
The only human interaction he attempts to make is with Susan, the girl who used to go out with Michael before he died.
Without any warning, Charlie walks up to her and says, "Do you ever miss him?" (4.1.18).
It's not a surprise—or very nice—when Susan's friends call Charlie a freak.