PALO ALTO, Calif. (CNN/Money) - "Why do we keep investing?"

The double-entendre query scrolled across the small screen Sunday night on, of all shows, "Sex in the City." Sarah Jessica Parker's amorous-affairs columnist Carrie Bradshaw is musing, of course, about relationships.

But in the solidly funny 30 minutes, high finance is a recurring theme.

These HBO writers have amazing timing. In an episode that had to have wrapped eons ago in television time, "Sex in the City" landed a show that perfectly captured the investor zeitgeist, with a handful of finance-related plotlines, including Carrie ringing the opening bell of the New York Stock Exchange.

CNNfn's Allan Chernoff compares past rallies and helps investors understand why this might be the beginning of brighter days for investors. Play video (Real or Windows Media)

Coincidentally (or was it?), the New York Times asked nearly the same question that Carrie did in a page-one story Sunday morning on the return of the individual investor.

In the pretend world, "Sex" character Samantha Jones gets a biotech tip from a fetching young financier. In the real world, the Nasdaq is up 20-plus percent year-to-date and Chinese Internet stocks are up by triple-digit percentages.

All of which are signs (potentially scary ones) that after three years of pain, the individual investor is back.

So why do we keep investing?

Well, for starters, we keep investing because it's the best way to build wealth. And there's absolutely no denying that when the S&P 500 index is up 13 percent for the year, it feels plenty good.

So feel good. About being up. About investing for your future. But don't forget the downside.

In the New York Times, the perma-bears who warned of the last downturn (in 1997, 1998 and 1999 before finally being right in 2000) are warning that this rally may well be a bear-market rally.

We've certainly seen them before. Even at the S&P's elevated level, it's up barely 1 percent in the past 12 months. It seems odd now, but that means the market was "up" at this time last year, too.

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I've been thinking almost nonstop about what to do about my own investments lately. My personal bottom line has grown because I never stopped investing in my broad-based mutual funds.

And it certainly helps that the shares of my employer and parent company of CNN/Money, AOL Time Warner (AOL: Research, Estimates), are up, because I own shares both outright and in my 401(k).

And yet I feel like this market has gotten ahead of an economic recovery that's far from certain. Part of me wants to play defense. The other part of me wants to be on the party train. Feel familiar?

So what am I doing? The portion of my savings I might need in the next 12 months is in a really safe place. The rest is in the market, including a higher dollop of bonds than I had in 1999.

If the market falls, I'll be sad for that portion of my assets. But it won't affect my plans. If the market keeps soaring as the economy dictates further gains, I'll happily ride the up elevator with everyone else.

Why do we keep investing? Well, what's the alternative?

Adam Lashinsky is a senior writer for Fortune magazine. Send e-mail to Adam at lashinskysbottomline@yahoo.com.

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