Last week, the Dow Jones Industrial Average (DJINDICES:^DJI) and broad-based S&P 500 (SNPINDEX:^GSPC) hit all-time highs, surpassing the psychological 15,000 and 1,600 mark, respectively. The Nasdaq Composite (NASDAQINDEX:^IXIC), while well off its all-time high, still hit its highest mark in more than 12 years.

The story is the same no matter where you look: The economy, the jobs picture, and housing are all improving. The U.S. economy only grew GDP at a 2.5% clip in the first quarter, but that's up from just 0.4% in the fourth quarter. Furthermore, the unemployment rate clocked in at a five-year low (7.5%), jobless claims hit a five-a-half year low on a seasonally adjusted basis (323,000), and the Case-Shiller Index, which measures home prices in the 20 largest U.S. cities, rose 9.3% over the year-ago period.

You might consider these figures irrefutable evidence that we're in the midst of a steady recovery that could lead to many years of sustained growth for all three indexes. However, a recent Gallup survey would completely refute that notion.

Released on Wednesday, Gallup randomly surveyed 2,017 adults across the U.S. and discovered that only 52% directly or indirectly own stock. This is the lowest reading since Gallup began tracking this poll in 1998 and continues a precipitous downtrend witnessed since 2007, when stock ownership was at 65%.

Percentage of U.S. adults invested in the stock market. Source: Gallup poll.

It also adds fuel to the fire that this rally from the recession lows may be among the most hated ever.

What's behind investors' lack of faith
The way I see it, there are four key reasons investors have significantly pared their stock holdings in recent years.

1. Companies aren't instilling trust with investors
The biggest problem with this rally is investors really don't have any reason to trust many of the nation's largest companies. Ask around and most people will tell you they didn't particularly care for their bank before the financial crisis. Ask them how they feel five years removed from that crisis, and most people will talk your ear off with vitriol for their bank.

Take Bank of America (NYSE:BAC), for example. Bank of America settled a long-running dispute with MBIA last week over mortgage-backed securities for $1.6 billion. If this had been but one or two settlements that Bank of America was settling relating to the financial crisis, investors would probably understand. But we're not even in the same ZIP code when it comes to settlement and ongoing lawsuits. Bank of America settled for $10.8 billion with the U.S. Justice Department and 49 state attorneys general over its mortgage servicing practices, forked over $8.8 billion to Fannie Mae, and faces an additional $5 billion to $15 billion in private investor lawsuits -- and that isn't even the half of it!

How can you feel confident about investing when the backbone of the financial sector is still such a mess?

2. Technology has changed things
Sure, the Internet has been around for 20 years, but in terms of what the Internet can do for investors and institutions, it has evolved dramatically over the past decade. The access to almost instant news and data via brokerage research centers, public portals like Yahoo! Finance, and even social media have changed the way news is disseminated. Worse yet, it's evolved investing into trading.

With news available at our fingertips, and institutions able to use algorithm-based programs to trade fractions of a penny faster than the blink of an eye (often referred to as high-frequency trading), the average holding period for owning stocks has dropped from eight years in the 1960s to less than a week, according to LPL Financial's Jeff Kleintop. People are easily swayed by market-changing news that wasn't easily disseminated even 10 years ago. With the market being ruled by traders instead of investors, it's no wonder many of the remaining long-term investors have little faith in this rally.

3. A wealth transfer is under way
Arguably, there isn't a group of individuals that took it on the chin worse during the recession than baby boomers. When stocks peaked in 2007, 65% of all Gallup respondents reported owning stocks in one form or another. Of those who participated in the 2008 survey, the 30-49 age group and the 50-64 age group were listed well above the average, with 72% and 71% affirming that they owned stock. Five years later, these figures had fallen by 14 percentage points and 10 percentage points, respectively.

What this means on a larger scale is that with many baby boomers nearing retirement and just too untrusting of the market, stock ownership's future is slowly being turned over to a younger generation. That's problematic because, as I noted previously, this is a generation that's been raised by social media and cradles technology to their very core. They often lack the patience to invest over the long haul and are easily swayed by the allure of a quick buck.

Without the market-calming presence of the boomers, the remaining long-term investors have viable reasons to be skittish about continued market appreciation.

4. It all comes down to valuation
Finally, it could be something as simple as valuation. We've had plenty of amazing runs higher in the history of the stock market, but few have added more than 130% in four years, as the Dow Jones Industrial Average has since its March 2009 lows.

While still inexpensive relative to the past two decades, the S&P 500, which makes up a good mix of 500 components across a myriad of industries and sectors, is valued at roughly 19 times earnings. Just 17 months ago that P/E was only 15. U.S. GDP growth has been good, but it also hasn't been anything to write home about, which could have investors leery about putting their money in one spot for the long run.

Also, certain technology stocks continue to act like the poster children for the chaos that caused the Internet bubble in the early 2000s. Take LinkedIn (NYSE:LNKD.DL), for instance, which offers a disruptive skill tool and job database for connecting with professionals online. While revolutionary and profitable (something many Internet companies of the late 1990s and early 2000s failed to achieve), LinkedIn trades at more than 80 times next year's earnings, 20 times book value, and nearly 18 times its total sales -- yet traders keep bidding it higher. Long-term investors have heard this song before, and they may want no part of it!

An ominous warning
A Gallup poll is certainly not an exact science, but it could forebode that all isn't as well as the numbers would appear with the three major U.S. indexes at or near their yearly highs.

Where do you stand? Has the recession changed your ownership habits any? Share your thoughts in the comments section below.