What might those reasons be for Clinton? Below are five conceivable hurdles. Let me qualify, though, that I still think it's much more likely that she will run than not. But any one of these things could be a significant obstacle in her decision-making process.
1. The prospect of losing
It seems like we've already discussed Clinton's inevitability ad nauseam, but that inevitability really only applies to the Democratic primary. Her odds in the general election are headed toward being a 50/50 proposition -- and getting worse.
Although Clinton's approval rating reached upward of 60 to 65 percent as secretary of state, her favorable rating in the days since she stepped down has steadily fallen -- so much so that most recent polls show her under 50 percent (although still more positive than negative). That's pretty middling territory.
Here's polling of a Clinton match-up with Jeb Bush:
And Sen. Rand Paul (R-Ky.):
And Rep. Paul Ryan (R-Wis.):
She leads all comers, but she's also better-known than all of them. She's also hovering around 50 percent and losing ground.
None of the charts above, we would add, include the new Quinnipiac University poll, which shows Paul and Bush trailing Clinton by five percentage points, and Ryan within four. Only against Sen. Ted Cruz (R-Tex.) does Clinton get more than 46 percent of the vote.
And all of this comes as President Obama's brand continues to fade. Running on the heels of an unpopular president of your own party who is leaving office is hardly ideal. Ask John McCain.
For Clinton, she has to decide whether it's all worth it to run and have basically a 50 percent chance (or less, including the primary) to become the next president. The country is closely divided, and Clinton has long been a polarizing figure. She should expect nothing less than a very tough campaign.
The age thing is always a difficult one to discuss, but if Clinton wins in 2016, she would become, at 69, the second-oldest president ever to assume office. Ronald Reagan was 69 in 1981, but was closing in on his 70th birthday.
She and Bill Clinton say she is healthy, and save for an incident in 2012 in which she fell and suffered a concussion and subsequent blood clot, there's little reason to believe otherwise. But even Bill Clinton has acknowledged the hurdle that is health, which he calls a "serious issue."
Perhaps as important for Clinton is the fact that she spent three straight decades in public life before taking the last 22 months "off." She also spent about half of those three decades under the intense national scrutiny as first lady, a 2008 presidential candidate and secretary of state (and that doesn't include her eight years as senator of the third-largest state in the country, New York).
Signing up for another campaign means committing to six years (if you win) of campaigning and serving, and as much as a decade if you want to serve two terms. That would bring Clinton to a quarter-century in the national spotlight.
Clinton clearly has a motor; she wouldn't be in this spot if she didn't. But getting involved in another campaign is a huge investment, and one that very few people can afford to enter into lightly.
Whatever you think about what happened surrounding the deaths of four Americans in Benghazi, Libya, in 2012, it certainly wasn't Clinton's proudest moment as a public servant. She even acknowledged earlier this year that the attacks were her biggest regret as secretary of state.
She has testified before Congress about the episode, famously intoning about the motivation for the attack: "What difference, at this point, does it make?" (Side note: Coming to numerous 2016 attack ads near you.)
Republicans pilloried the Obama administration for initially and repeatedly saying that the attacks were random, when in fact that turned out not to be the case. Again, that wouldn't be helpful for Clinton in 2016.
Polling suggests that a slim majority of Americans are both open to further inquiry into Benghazi and also doubt Clinton's statement that she wasn't aware of requests for further security for the Americans in Benghazi before the attack.
Clinton might think this is old news, but the moment she becomes Candidate Clinton is when she'd have to answer more questions about Benghazi. The GOP will make sure of it.
If you think the scrutiny of the Clinton family's finances has been intense, wait until she announces she is running for president.
Case in point is a Washington Post article over the holiday weekend, by Rosalind Helderman and Philip Rucker, detailing the behind-the-scenes action surrounding Clinton's March 5 speech at UCLA. And then there was the scrutiny of her speech at UNLV, for which she was paid $225,000.
Clinton, in case you need reminding, hasn't handled questions about her finances with aplomb. There was the time she said that she and her husband were "dead broke" upon leaving the White House and struggled to pay mortgages on their multimillion-dollar homes (yes, plural), then doubled down on that claim, and also differentiated her family from the "truly well off."
Rucker back in June detailed some grumbling about Clinton's ability to appeal to an increasingly populist Democratic Party, and it's a fair question to ask whether she's the right fit for her party at this point.
Whether or not someone like Sen. Elizabeth Warren (D-Mass.) were to run, Clinton's wealth and her family's ties to Wall Street will be at issue.
Also worth noting: The moment Clinton announces is when the $200,000 to $300,000 speeches probably would stop. That's a lot of future earning potential she would be giving up.
5. Questions about the 'Obama coalition'
Conventional political wisdom has it that Democrats are well-situated for presidential elections, while Republicans excel in midterm elections.
A more apt statement might be this: President Obama is well situated for presidential elections, while Republicans excel in the midterms.
The fact is that Obama got young people, blacks and Latinos to vote Democratic unlike anyone before him; that's why we call it the "Obama coalition." But the question is whether that revolution is transferable to someone not named Barack Obama -- i.e. would young people, African Americans and Latinos really get as excited about Clinton in 2016 as they did about Obama in 2008?
Former top Obama campaign strategist David Plouffe summed it up well recently in an interview with the New York Times's Jonathan Martin: "We shouldn't just assume that the Obama voters will automatically come out for Democratic presidential candidates."
There has been plenty of chatter about Clinton expanding the Democratic Party's appeal to white working-class voters and maybe even competing in states such as Arkansas and Kentucky -- we are dubious, to say the least -- but her clearest path to victory is keeping the Obama coalition intact and taking advantage of the blue team's need to win relatively few of the swing states that already exist to secure a victory (rather than trying to add more).
And if she doesn't think she can do that, her path to victory is even narrower than some of the polls above suggest.