Buying and holding passive investment vehicles like the S&P 500 is a crude approach to what an investor really cares about for his or her own personal circumstances.
While I don't necessarily agree with the entire statement it is the case that personal circumstances trump everything else. There are several points here.
In no particular order...tolerances for normal stock market volatility is a big one. The building block here is probably during normal market corrections and bear cycles and then realizing 2008 was not normal but that type of decline can happen every so often. The answer here is proper asset allocation, some sort of defensive strategy or a combo of both.
Personal savings rate is very important and at least partially dictated by volatility tolerance. Someone who no matter what cannot tolerate more than a 10% decline is probably going to have a low exposure to equities. This is ok but the tradeoff is a higher savings rate to compensate for forgone growth, the expectation of working longer, a more modest lifestyle than probably hoped for or a combo of all three.
Likely health or lack thereof will be a factor in personal circumstances. My firefighting experiences show that aging and health are very individual things. On medical calls I have encountered people in their 40s and 50s with many health problems. I've also fought fires with guys in their 60s and 70s who were just as physically capable as guys in their 20s. This can be a serious reality check for people and I do not minimize the difficulty but these sorts of issues are a part of a comprehensive financial plan.
People also need to take inventory of their beliefs and how they should factor in to their finances. The easy example is leaving money to children. I've met people who believe it is a moral obligation to leave money to their children and others who believe it is absolutely the wrong thing to do. There is no wrong answer, just a personal belief and there are others too to sort out.
People who are lucky enough to live below their means are going to have an easier time with saving, absorbing financial shocks and being able to take what the market gives. As an example, someone living a $40,000 lifestyle on a $100,000 income can have a very high savings rate, will have an easier time finding income to cover their expenses if they lose their $100k job and is less likely to have to get a certain return. What I mean by that last one is that in the last ten years it was very unlikely to have averaged 10% per year as the SPX averaged 5.5% so anyone planning to retire in 2012 and needing 10% in that last ten years now has a problem.
Figuring all of this out can and hopefully will make investing a little easier, psychologically, as a more suitable portfolio can be constructed such that it takes on the proper amount of risk and volatility.
And as a friendly reminder, if you live in one of those weird states with daylight savings time, don't forget that the time changed overnight (old joke as I live in one of the two states without daylight savings time).