Although Karl did not want to do this, he had to agree with his fiancée. The metaphor she used got to him because it spoke the language of racing.
Many parents instruct their children not to drive too fast. Mothers' hearts trembled as their sons became racing drivers. The sport was safe, yet their hearts trembled. Some have explicitly asked that their sons withdraw from the sport. There were also those that put kids out of their dreams of racing early enough so that they would never try their chances. These boys never got behind the wheel of a racing car, and perhaps the world will never see a true champion who turned into a bank clerk because of his mother's fears.
Karl acted exactly like these mothers. The problem was, he wasn't Steve's mommy. So why the hell was he interfering so much with his private life?