Courage: There's something fishy goin' on here, or my name is Stinky Looloo, and thank goodness it's not.
Talking Fish: There's no such thing as perfect, you're beautiful as you are, Courage. With all of your imperfections, you can do anything you want to do.
Freaky Fred: Goodbye, dear aunt, I'll miss your farm. And Eustace's eboliant charm. And farewell, Courage, what's the harm? If I was slightly... naughty. With love, Fred.
Muriel: Courage, you know I can't hear without my glasses.
Muriel: I only wanted to be a good neighbor.
Eustace Bagg: You should have let the neighbors get extinct and make me coffee.
Courage: Oooh, I know I'm not gonna like this.
Answer: Eustace is a rather two-dimensional antagonist character. He is just a mean-spirited man who takes out his hostilities on a vulnerable dog and resents his wife having adopted him.
raywest ★
I thought the reason had to do with the attention Courage receives from Muriel.