Low Inflation (by traditional measures such as cost of cheese, buns and hotdogs) which has given those who control fiscal and monetary policy license to:

1. Engage in unprecedented multi-trillion quantitative easing programs all over the globe (financing debt). More cash pushes stocks up as the low yields everywhere else (like bonds) favors stocks. Money has to go somewhere when you are a pension fund manager or other money manager and you HAVE to have short term yield.

2. Keep Low Interest Rates (even negative yields?) for a long period of time, making debt reaaaaally historically cheap, allowing companies to buy back stock at all time highs with cheap debt. This increases Stock Prices as it increases Earnings Per Share this pushes stocks up.

Also consider:

3. Paradigm shift of the overwhelming majority of stocks are bought via huge swathes of money in Index Funds. This is dumb money. Buy stocks (which is ownership in (the profits of) a company). But price discovery is further destroyed because the robots buy on autopilot. The money has to go somewhere right? So it buys stocks that otherwise have a low volume of daily purchases (like $5 million) at a much higher volume (like $100 Billion). Price has nowhere to go but up while the bulls are in control and money is easy.

4. We came off the best time to buy stocks and houses in our lifetime (2009-2011) as there were many high quality assets and businesses at really low prices. Now we are higher than 2006. We're actually barely over 1 double since the peak in 2007. But we've more than quadrupled since the bottom in 2009.

In general, credit cycles run about every 7 years. We're on a record run. At some point, things will reverse.

Debt for Governments. Haven't checked in a few months but last I checked there was something like over $15 trillion in negative yeilding debt. This has pushed banks to take on more risk in places like Argentina and Turkey. Places that have a large default risk of not being able to pay back their dollar-denominated debt with inflation about to get out of control. Many countries, including the US, run deficits that are larger than their actual GDP.

Debt for Companies. Management can also use debt to enrich themselves, buy companies, jets and other things because they have a huge line of credit granted to them. In the long-run management is gone, so with few exceptions, self-interest wins over longterm shareholder interest. This is absolutely happening. Responsible management teams have been punished by the market in many cases.

At some point, it's likely the tide rolls out and we'll find out who was swimming naked. Then, many average companies, below average companies and bad companies will face major debt problems, which leads to bankruptcies, which deepens recession. Stock prices will be under major pressure. When Indexes start the trend of auto-sell (at any price) I expect a huge swing of the pendulum the other way. Those with cash, like Warren Buffett, will absolutely start deploying that cash when there is blood in the streets and things look like they will never get back to normal. I say raising cash is a prudent thing to do now.