One of my family members causes me more pain and suffering than the entire rest of my family combined. My two best friends are the source of more happiness to me than all my other friends put together.
The Pareto principle (also known as "the 80/20 rule") is the idea that, in many domains, 80% of the causes generate 20% of the results and 20% of the causes generate 80% of the results. That is a 16× differential in efficiency. The 16× differential is a lower bound. In software, 1% of the code tends to consume more than 99% the CPU time.
If 20% of my work produces 80% of my desired outcomes then that implies:
Rule #2 is an undeniable mathematical corollary to the Pareto principle. If a domain is Pareto-distributed then Rule #2 applies.
The more extreme a distribution is, the lower the threshold for Rule #2. If 10% of the causes generate 90% of the results then spending more than 10% of my time on an activity implies I am spending too much time on it.