I believe the fundamental reason is because of hedge funds, large automatic quant funds, cheap fed capital, and the easy flow of huge amounts of money. Basically if there is an edge anywhere in the market, the large funds will immediately pump tons of money into that sector, and that edge will go away. They move funds very quickly and they move enough to shift the profit margin, and they keep doing it until it's no longer desirable.
Put another way, why the hell would a company want your capital? The reason why investments return a profit is because somebody needs your money for their company. In exchange for your $10k they pay you back some interest. But when the fed funds rate is near 0%, the big banks can suck out any amount of money they want and give it to whoever needs capital. So why the hell would any company deal with paying a return to small individual investors when they can just get capital so easily from the big banks? The answer of course is that they don't. When capital is cheap, the rate of return on investments rapidly goes to zero.
I believe that the last 100 years or so (and mainly from 1950-2000) has been an anomaly - a period when individuals could easily get a nice return from investments, and that there's no reason to think we will have this in the future. In the last 100 years stocks have return 3-5% after inflation, which roughly tracks GDP growth (it's no surprise; total stock market value and dividend yield both directly track GDP growth).
Now, obviously if you look at the market recently it doesn't look weak. But I believe that we are in a very large bubble. It's an unusual bubble because it appears to be affecting almost every type of investment - including things that should normally track opposite of each other. That's very strange. A quick rundown of what I think is happening :
Stocks : I believe the current stock bubble is driven by these factors : 1. massive free money from the Fed has to go somewhere; when it turns into buy orders, stocks have to rise (obviously in theory the traders could say "no I don't want the free money because I don't see anywhere profitable to invest it", but LOL that doesn't actually happen). 2. massive free money makes the economic numbers look better than they really are. 3. under-counting of inflation and unemployment makes the economic numbers look better than they are. 4. individual investors and such have tons of money in 401ks and such and don't know what else to do with it, so it goes back into the market. I think everyone sane should realize that stock valuations right before the Great Recession were way out of whack - and values are right back to that level, which makes no sense since nothing good has happened in the economy, in fact we're fundamentally much worse now than before the GR.
Gold : historically, from 1850-2000, gold returned slightly *below* inflation, eg. real return was negative. The only period when it returned well was during the massive inflation era of the 70's (and the last 10 years). Gold should generally track against other economic trends. Yet gold has done very well for the past 10 years, even during times that stocks have gone up and inflation is nominally low. I don't think this makes any sense and I believe that gold is in a massive bubble. If nothing else, when you have people like Glen Beck touting gold, and all the "Buy Gold Now!" web sites pushing it, there's obviously some bubble aspect, and it's all very reminiscent of real estate 5 years ago.
Real Estate : residential & commercial are both bad. Oddly, REIT funds are up to levels matching before the GR, to me this clearly indicates they are at bubble levels. Residential values in most of the US never fell as much as they should have, and tons of the MBS's that the Fed continues to buy up are not worth what they claim. Some people think commercial real estate is due for a big crash; I'm not sure a crash will actually happen, but if it doesn't it will mean a weak market for many years.
Bonds - oddly bond values have gone up at the same time as stocks. Treasuries in particular have been hovering around zero yield, which only makes sense if you think there is a real risk of massive corporate failure or currency crisis or other reasons why you wouldn't just keep your money in a bank returning 1% or a AAA corporate bond returning 4%. (BTW I was always a little confused about the whole idea that bond values rise when yields drop, but really it's very simple. Yields are the return of newly issues bonds. If you have an old bond that returns 4% and the newly issues yield is 1%, then people who want a 1% return could buy your old bond at a 3% higher price and get the same yield ; that is, prices move to make the final return at maturity the same). Bond prices should generally track against stocks, but they've both been doing quite well since the GR.
I don't see anywhere good to put money. And furthermore I do think there is some risk of heavy inflation, so sitting with cash in the bank isn't great either. (a few ideas : I believe gold is so inflated it might actually be a good short; it's also possible that real estate in some areas might be a good investment, at least real estate is a hedge against inflation even if it doesn't return anything; unfortunately Seattle is one of the places where real estate hasn't fallen as much as it should which will lead to long term price stagnation, or even declines in real dollars).