This quote really puts investment performance into perspective if you have 10, 20 or more years of future earnings to add to your retirement assets. Your "human capital" (what you make) can be very important to your overall financial health.
From Nobel Laureate Robert Merton (Morningstar video clip created in 2011):
“…If we only focus on the financial piece and we say we put a 100% of that in equities, when you just look at what happens to that, it looks very risky. We say to ourselves, "We had a decline of 30%; equities have lost 30%."
But if we look at it more holistically and say, "Really it’s not that bad. They only have 10% of their retirement assets in this, because the other 90% is future contributions," then saying a decline of 30% is really only 3%....”