Marcus Lemonis of CNBC’s The Profit says that the best time to bring in a partner for a business is right at the start. You need someone who thinks differently from you and who sees the things you miss. A good partner challenges you and debates with you. He or she will keep you in check and help you solve problems that your startup will inevitably face. A person who says he doesn't want or need a partner is a person who thinks he knows everything...and probably won't listen to suggestions from investors.