One, because sarcasm sounds like sincerity, but upon deeper inspection you can detect that it's actually satirizing the sincerity. Two, because her father was not stable and could never hold down a job, but would always make sarcastic jokes about it. Because he could never hold down a job, Mertle never knew if the next meal was going to come. So when she reached adulthood she ate all the time, and in large quantities, because she had constant dread that food would go away. She did not like her dad, and she did not like Bub. But she was married to Bub, because Bub reminded her of her dad. And her dad was the only example of a male she knew. Life had really created an unpleasant equation for her.
Bub loved Mertle because she was humorless and fluctuated weight a lot. He liked that she fluctuated weight because he was attracted to all different types of women, so if her body weight changed it would be like he had a big woman one night and a slim woman another. He liked that she was humorless because it reminded him of his humorless mother, who was very stern and that caused him to rebel by developing a keen sense of humor. Also Mertle's lack of humor made him the funny one, which he liked.
"I'm instinctually drawn to you, but can't stand being around you, yet I fulfill your every need. That's not fair!" said Mertle.
"You're complete anguish to be around, yet I absolutely love your company." said Bub, with complete sincerity.
"Are you being sarcastic?" asked Mertle.
"No." said Bub, sincerely again.
"It sounds like you're being sarcastic." said Mertle.
"I know, but I'm being sincere." said Bub, still sincere.
"It's hard to tell because you're so often sarcastic!" said Mertle.
"It's something one might say when being sarcastic, and I know I have a tendency to be sarcastic, but I'm breaking sarcastic-character to let you know that you're a rotten horrible woman, and I absolutely love you, you fat-skinny-fat-skinny-angry-tough-crowd-whiny-scowling ogress."
"I believe you. Thank you, darling, for not being sarcastic for once. I actually like what you're saying because it's not sarcasm, I hate sarcasm." said Mertle, lovingly.
"Really?! I thought you loved sarcasm." said Bub, sarcastically.
"Dammit, I hate your fucking sarcasm!" screamed Mertle.
Bub and Mertle had a baby. It was hard to determine what the baby would grow up to be like, personality-wise, and what combo of the two parents it would exhibit. They were so uncertain, they brought in a mathematician and explained their whole personal situation, so he could assess how the child would turn out, just for no reason.
The mathematician's answer was reasonable and believable, but he was real smug and pleased with himself. Gross. Bub and Mertle could tell he was going to go around bragging about his findings and using them to schmooze up the mathematician ladders. They kicked him out.
One time during the holiday season Bub's cold humorless mom came over to spend time with the family. She was just like Mertle. Mertle's sarcastic unstable dad came over too. He was just like Bub.
Each parent was widow and widower. Bub's mom and Mertle's dad had real strong sexual chemistry so they had sex with each other. The grandparents were too old to have new babies. But they tried to figure out what another baby by them might be like, personality-wise. They couldn't figure it out.
"Why don't we call the mathematician again?!" said Bub, sarcastically. Mertle's Dad laughed. The ladies scowled.