What this rule checks
Your Mars is in the 1st house. Mars is about drive and courage. The 1st house is about you, your body and how others see you. Source: Phaladeepika, chapter 8 (a classic text), slokas 8 to 10. Notes beyond the classic text are the author's own modern reading. Classic texts describe this placement harshly. Read it as a tendency, not a verdict.