By default, there's no / little reason not to invest (most of) your money, other than necessity. Even those who understand it, sometimes are not doing it though.
"Look at the stock market returns. It's making record highs for the last n years. I'm not gonna to invest - it's too risky."
There are a couple hidden assumptions in here:
- You don't need to invest in the stock market exclusively. There are other, publicly available asset classes that you can mix in with stocks. You actually gain a lot from it!
- If you think in terms of a portfolio, then you can much reduce your timing risk. You don't need to find the perfect time to invest. Not being invested probably will cost you more.
"Look at the stock market returns. It's been in a free-fall for the last n months, and the outlook for the economy is horrible! I'd be crazy to invest now!"
Historically, these were the best times to invest into the stock market.
The thing is that if you only invest into a single asset class, like stocks, timing matters a lot - and people usually get it wrong. I'm sure I would.
If you don't want to make sub-optimal decisions, try to be a prophet of the world economy, lose a lot of money when you're wrong (inevitable, if you live long enough:), it's worth looking into diversification.
To further reduce your timing risk, you can also look into Dollar Cost Averaging.