Risk. Reward. Take more of the former. And over time, you get more of the latter. But you also get volatility. And investment securities, particulary stocks or equities, are risky and volatile. And rewarding. Check out the chart of the S&P 500 since the late 19th century.
Peaks. Valleys. Peaks. Valleys. Peaks. It goes up a lot and down a lot. But over time, well...it goes up. In fact, over time, the market has gone up about 10%, give or take a year, but with long periods where it did way better, and long periods where it did way worse.
So you can’t invest in the stock market with a short-term view, really. It's like navigating a ship with a magnifying glass instead of a telescope. If you’re going to take on the risk of the market, you mitigate a lot of that risk by just committing to own your basket of stocks for a very long time. If you do, and history continues to repeat itself like a bad Thai food dinner, then you’ll double your money about every 7 or 8 years.
The bond market is a different animal. Yields skyrocketed as the Jimmy Carter administration tried hard to fight and stomp out inflation. And it did. And since then, bonds have been on a long, slow ride down to the modern era, where yields are almost nothing. It's unprecedented to have such "free" money, but that’s where we live in today’s world, with governments desperate to stimulate inflation so that they can pay off their debts easier.
Over the decades, bond yields have come down, and today, the 10-year T-bill yields about 3%; corporate, modestly risky bonds yield about 5%, and they are way safer than similar stocks. That is, bonds are way safer than stocks in the short run.
Only a very small handful of bonds as a percentage of the total out there ever lose money by not paying their interest and principal, whereas stocks lose money all the time.