according to candy why had curly been worse lately

He thought his wife was cheating on him.

According to Candy he got married;

“Seems to me like he’s worse lately,” said the swamper. “He got married a couple of weeks ago. Wife lives over in the boss’s house. Seems like Curley is cockier’n ever since he got married.”

Yes, he hates being married. It means he actually is expected to communicate with somebody. He considers his wife a possession. Being the only girl on the ranch, and terribly lonely, she often wanders which makes Curley insanely jealous.