Why was Pedro’s father angry with him?

Pedro's father was angry that he agreed to marry Rosaura when he was in love with Tita.

"Why did you do that, Pedro? It will look ridiculous, your agreeing to marry Rosaura. What happened to the eternal love you swore to Tita? Aren't you going to keep that vow?"
"Of course I'll keep it. When you're told there's no way you can marry the woman you love and your only hope of being near her is to marry her sister, wouldn't you do the same?"
Nancha didn't manage to hear the answer, Pulque, the ranch dog, went running by, barking at a rabbit he mistook for a cat.
"So you intend to marry without love?"
"No, Papa, I am going to marry with a great love for Tita that will never die."
Their voices grew less and less audible, drowned out by the crackling of dried leaves beneath their feet.