There are plenty of great responses here, but I will go ahead and throw my hat into the ring.
The first chapter of The Intelligent Investor by Benjamin Graham spends some time differentiating the difference between a speculator and an investor.
To my pea-brain, that has about 1.5 to 2 years of actual experience and self learning stored, whenever you get into subjects like stock picking, options, commodities -- whatever it might be -- you're speculating that those investment vehicles will bring you a return. Hopefully one that will beat the market. To what degree, anyone could guess.
No one knows how to beat the market consistently, how to pick winning stocks consistently, or what the price of cattle will be in the future. If this happens to be your area of expertise then I say go for it. But at the end of the day, I'm sure you'll find more losers than winners. Maybe some do, but as your average guy I do not have those kinds of resources.
However, whenever you deal with known asset classes like the the stock market as a whole, or the return on a bond, or the even the return from a rental house, you deal with something that's more concrete. Instead of spending countless dollars and hours trying to beat the market, why not be happy with matching it? You're investing. If you read the jcollins stock series above or read any number of books (The Bogleheads Guide to Investing) you can find lots of evidence pointing to two things
1) You really can't beat the stock market consistently
2) The market will always go up
So in my head, I don't worry about picking a fund that will beat the market, or whether I should buy Amazon or Exxon to carry me to retirement. All I know is that I am 34, and I want to have a 75% equities, 25% Fixed Income allocation. Our retirement accounts consist of 3 Vanguard funds -- The total stock market index, the International Stock Market Index, and the Total US Bond Market Index. All I have to worry about is contributing to those three funds and making sure my assets are allocated. As I get older I will move more money from equities into fixed income. That's it.
I know this can be an overly simplified view of things but that's kind of the framework for my investment philosophy. There are others that are successful doing other things, but this is what works for me right now. If you're not an expert like me, hopefully this can help.
Will