Why do many business owners prefer corporations over other forms of business organization?

Petra contracted to paint bret’s house for $2,000. after beginning the job, petra realizes that the house is really quite big, and she’s not going to make enough profit, so she tells bret she wants another $500 to finish the job. bret doesn’t want to pay more, but he’s afraid that if she walks off the job, he’ll have trouble finding someone else to finish it, so he agrees. is bret legally obligated to pay the extra $500?