Okay, let me just be simplistic and quasi-quantitative. Let's imagine most of the world start working at 20 years old, work 5 days a week, retire at 60, live till 80, get married and have 2 children in each family.

So, you would spend 5/7 of 40 years of your live earning money, 2/7 of 40 years and 7/7 of 20 years spending money, while your kid spends 7/7 of his first 20 years, spending your money. So,

   (5/7)*40 : (2/7)*40 + 20 + 20

=>    200/7 : 360/7

=> (working : spending) = (5 : 9)

Say A and B have equal need. So A would serve B, and B would serve A. So when A works, B enjoys, and when B works, A enjoys. So A would work 7 hours in 14, and B would work another 7 in 14.

Yet here, you would work 5 hours in 14. So someone somewher is working 9 in 14 - afterall, someone's gotta give, rite? That would be 9/5 as much work as you. But if you work 5 days a week, how can that someone be woring 9 days a week?

Ah, that's because you can make more than 1 person happy at the same time. If you could make 2 people happy in 1 hour, then you can work 1/3 of the time and everyone will be happy. Ah, maybe that's the strength in numbers - and why personal service is so expensive...

okay, maybe this sounds like rubbish :P