Perhaps the furry cartoon guy has a special ability to understand the language of trees and sees them as friends to talk to. It could also be that he's seeking wisdom or inspiration from them.
It could be that the interaction is to provide some sort of closure or resolution for the character. Or it might be used to raise philosophical questions about death and the beyond.
Maybe he wants to play a prank or get something secretly without being noticed.
Maybe it's for adding more fun and entertainment to the cartoon. It makes the dog more interesting and engaging for the viewers.
Maybe she's frustrated or trying to express her annoyance. That's a common gesture when someone wants to shut down a conversation.
Maybe the kid is just an outgoing and chatty character. It could be part of their personality in the cartoon.
It could be that he's really frustrated or angry. Sometimes characters scream to express those intense emotions. Or perhaps something unexpected happened that shocked him.
Maybe he's just in a good mood and has something to be happy about.
Maybe the big guy is having a bad day and takes it out on the small guy.
Maybe it's part of its character design to give it a unique trait.
Maybe it's to be cute and appealing to the audience. That kind of voice makes it more endearing.