So Charlie Brown’s troubles are not because he’s a loser, they’re because he’s shallow: He’s in love with a girl because of her looks. He wants to kick a football because his ego won’t let him fail (or learn that Lucy is a sociopathic liar). He is a child, but his dog has a much richer inner life.
I don’t think this explains the kite-eating tree.
The kite-eating tree is a metaphor for Charlie’s inner angst. Or that is simply one bad-ass tree.