What does not kill you...

... makes you stronger

And this is especially true for a fitness routine, provided:

• You are giving yourself adequate rest between your workouts
• Your nutrition is aiding the strengthening process
• Your workout program is holistic, i.e. you're not injuring one body part for the sake of strengthening another.  Examples of not following this principle include workout routines that cause RSI
• You are pushing yourself to the maximum each time you work out.  This is the principle of progressive overload.  There are exceptions though: one shouldn't train to failure when hitting the tendons (e.g. with pull-ups, chin-ups)

Yes, there are prerequisites to every fancy rule.