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The S&P 500 has averaged a 6.8% annual return after inflation for the last 140 years. That's enough to turn a $400 monthly investment into $1 million over a 40-year career. That's amazing, and it's for the taking.

But I love investing because of what happens in between. There's nothing else like it.

Stocks are up 225% since 2009, which is one of the best five-year periods in history. They're also up 53% since 2007, which, after inflation, is one of the worst seven-year periods in history.

So, are we in the middle of a blistering bull market or a drawn-out stupor?

How you answer that question depends on whether you're bullish or bearish on the market in general. There's almost no objectivity in the market. Just shades of whatever you want to believe, and how you want to interpret things.

That's part of what I love about investing. Nobody will ever solve the puzzle, or accurately answer what's going on. There are just opinions and feelings, which keeps things exciting. We'll be learning about markets forever.

I love investing because there is no such thing as normal. At every moment the market is in one of five distinct moods: Obliviousness, denial, self-righteous gloating, paranoia, and stark-raving lunacy. This also keeps things exciting.

Investing requires, more than anything, patience and discipline. But it attracts, more than anybody, the impatient and impulsive. This promises a great show -- a slow-motion car crash that you can watch forever.

You can make a decision today to use the money you saved yesterday to make yourself better off tomorrow. That's pretty cool.

Thousands of CEOs go to work every morning with the goal of making you – no matter your age, education, or background -- better off. That's even cooler.

Most people view stocks as owning a wobbling group of paper tickets. The smart people view them as owning stakes in real businesses.

Every day you own a stock that owns a dividend, you get paid a little more. A few bucks today. More tomorrow, and the next day. All for doing nothing. 

People always say, "There's a person on the other side of every trade, and that person thinks the opposite of you." But do they?

Maybe the guy selling shares to me needs money to pay his taxes, or send his kids to school, or buy a new boat. We might think the same thing about the prospects of a stock, even if he's selling and I'm buying. But I don't know. I'll never get to meet him. Maybe it's not a he, or a she. Maybe it's a computer. And maybe they do know more than me. Or maybe I know more than them. Isn't that just as likely?

It's like the world's biggest poker game. Buyers tempting you to fold, sellers wishing you won't. You can sit there counting your chips. Or try to outsmart the other guy. Play as many, or as few, hands as you want.

Investing gives you more insight to human behavior than probably any other field. That's because it's emotional, it's personal, it's political, it's cultural, and we have insane amounts of data to measure it all.

You can explain Warren Buffett's approach to investing to an eight year old, but no one has been able to replicate his success, at least to the degree he's achieved it. It's a challenge that keeps people fantastically intrigued. 

Two peopled shared the Nobel Prize in economics last year for their theories on market bubbles that are practically mirror images of each other -- one says bubbles can't exist, the other is famous for spotting them. This, too, keeps things fascinating. 

Volatility is a guaranteed part of markets. If markets weren't volatile, there would be no risk and prices would rise so high that a crash would become guaranteed. But even people who understand this want to believe that crashes are abnormal, dangerous, and need to be explained by pointing fingers at someone else. This, too, keeps it interesting. Markets will never, ever, be boring.

What other games of skill do you know where an orphaned secretary who lived in the woods and never owned a car dies with $7 million in the bank in the same year an Ivy League-educated MBA who runs the largest networks of financial advisors goes bankrupt?

None. You know of none.

Everyone in the stock market has an opinion. I do, you do. Most of them are wrong, biased, incomplete, or uninformed. Neither of us care. We're too busy arguing with each other to notice.

There are no other industries where people commit so much of their hard-earned savings into something they know and have thought so little about. This highlights how emotional this game can be. And why so many people distrust it.

The smartest people in the world have been humbled by it. Idiots have gotten rich off of it.

It will work wonders for you if you let it. It will destroy you if you allow it.

And it's available to anyone with a few hundred bucks to spare.

It's so much fun.

I love this game.

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More from The Motley Fool: Warren Buffett Tells You How to Turn $40 into $10 Million

Contact Morgan Housel at mhousel@fool.com. The Motley Fool has a disclosure policy.