Oaktree's Howard Marks writes one of the must read quarterly letters (embedded below). This one is a bit lengthy, including many extended quotes from mainstream sources, but there are a few tidbits that resonate soundly with me.
First of all, page 5, which describes the basic unsustainable characteristics common in many global economies. More importantly, Marks's conclusion, on page 17, where he notes:
bottom line: anyone who invests today in a pro-risk fashion out of belief in the recovery must be confident that he'll be agile enough to take profits before the long term realities set in.
I've used the term "reality" many times to answer the question "why are markets down today?" And Marks's quote perfectly describes why I'm not going to be majorly long stocks any time in the near future: 1) I think reality is much lower, and 2) I don't think I am agile enough to pass the grenade once the pin is pulled.