Why is the party scene so important to Charlie? After they call him a wallflower, they say “To Charlie” and toast to him- why does this make Charlie emotional?

While Charlie begins to participate more by attending the Homecoming events and by forming closer relationships with Patrick and Sam, he is also appreciated for his position as a wallflower - someone to quietly observes and understands. The guests at Bob's party validate Charlie's desire to understand; their acknowledgment that Charlie is able to understand what he sees is formative for Charlie, particularly because he is so obsessed with understanding others.