(CNN) Natural disasters are dramatic enough, and yet every time one hits, the Internet lights up with years-old pictures, out-of-context videos and, inevitably, the mascot of all fake social media posts: road shark.

If you don't know what road shark is, it's an image of a shark swimming on a flooded highway that rears its Photoshopped head every time there's a hurricane or major water-related event. The image, of course, is fake.

Believe it or not, this is a shark on the freeway in Houston, Texas. #HurricaneHarvy pic.twitter.com/ANkEiEQ3Y6

But why do fake posts get shared, time after time, disaster after disaster? There are a few reasons.

1. People have been trained to believe

Some people sometimes trip over one sticky notion: if it's on the internet, it must be true.

"We've all been trained to assume that what we see published must have gone through some set of filters and authentication," says David Weinberger, an internet and technology expert and senior researcher at Harvard's Berkman Klein Center.

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"You used to watch the news on TV and you were confident it had been vetted or whatever. It was not your responsibility to fact-check the news. Simply being published was a mark of authority."

Now that anyone with an internet connection can create and share content, that old assurance of trustworthiness doesn't quite fit. But the assumption that something can't be false because it's being shared and discussed somehow lingers on.

During Superstorm Sandy in 2012, a Wall Street analyst tweeted out a series of updates about flooding, destruction and power shutdowns around the New York City area. They were all fake, but the thousands of people who read the posts and retweeted had no idea.

And how could they, unless they were there experiencing it with him? They had no immediate reason to believe what he was relaying was false.

One of the fake tweets sent during Superstorm Sandy in 2012.

Even big names can be led astray by the power of trust and friendship. During Hurricane Harvey, Katie Couric posted a picture a friend had sent her, ostensibly of an alligator in the friend's driveway.

Look who wandered into my friend's Houston 'hood Thinking about everyone affected by Harvey and 🙏🏻 they r safe pic.twitter.com/BTuGLACiqg — Katie Couric (@katiecouric) August 27, 2017

However, when people pointed out the photo was actually from months earlier, Couric's friend's story seemed to unravel, and Couric called it a "double punk."

Mmmm. She's not answering. She lives in Houston but in nyc and someone sent to her...she sent to me-looks like a double punk! 😖 https://t.co/5RAFW8XJxt — Katie Couric (@katiecouric) August 27, 2017

"There are other indications of the trustworthiness of what's being said," says Weinberger. "For instance, the number of likes or shares can be an indicator of trustworthiness."

After all, the mistaken assumption goes, if a lot of people believe it enough to like or share it, it's probably true.

2. It fulfills a confirmation bias

Confirmation bias is the idea that we're more likely to consume and share information that fits with our already-held beliefs. It's another big reason why someone may be compelled to share information without doing their due diligence.

This often happens after terror attacks or shootings. People latch on to erroneous or made-up reports if the reports fit their assumptions about the situation -- or their political and moral agenda.

People are editing, and photoshopping my selfies as if I am one of the people causing the issues/problems in Paris. — Veerender Jubbal (@Veeren_Jubbal) November 14, 2015

"You are always going to believe that which makes sense to you," Weiberger says. "And when there's very little information, it is much easier to throw out ideas that seem plausible."

In Charlottesville in August, a photo of an apparent "antifa" protester attacking a police officer was widely shared by supporters of the white supremacist rallies. The photo was a fake. It was actually an image from 2009 that was manipulated to include an "antifa" logo on the attacker's jacket.

3. They want to be part of the conversation

As Hurricane Irma blows through the Caribbean, some Facebook videos of the storm's destruction received millions of views . The videos were peppered with well-meaning comments. "Blessings to everyone here," one person wrote.

The problem? They're not from Irma at all.

Similarly, fake forecasts and warnings became so pervasive this week that the National Weather Service had to issue a clarification warning people.

When something horrible happens, whether an act of God or an act of man, it's human nature to try and draw some sort of understanding from the chaos. It's human nature to want to inform and comfort. So, we share.

"There's social capital in doing that, too," says Weinberger. "That's one reason people will tweet things, especially if they think they are the first one in their circle (to do so)."

When you find a new or fascinating piece of information, "it's a sense that you are helping your community or your network make sense of the event," he continues.

"They feel, 'It's my civic duty to move it along.' Then it's not just social capital. It's, 'Here is something that will touch you or inform you.'"

4. Some just do it on purpose

For the most part, people who share fake posts online don't actually know they're sharing fake posts online. Those that do, well, there's not a lot you can say to make sense of it.

"That's impossible to answer," Weinberger says. "It can be pathological. (It can be) somebody who is so eager for affirmation. It can be trolling. Or somebody purposefully trying to disrupt the news."

5. Sometimes the truth really IS that bizarre

As a massive wildfire raged near Portland, Oregon, this week, the Beacon Rock Golf Course posted a photo of a group of golfers putting around as if there weren't a giant, angry, red inferno directly behind them.

Immediately, people were skeptical. The photo looked manipulated, some said. They debated and scrutinized; they couldn't bring themselves to believe it was true.

The photo certainly looked like fake, but it wasn't . Perhaps one reason people aren't more discerning when they come across something that's fantastical (but fake) is because sometimes reality is equally awe-inducing

Maybe, just maybe, in this age of misinformation, we'd have less fake stuff to deal with if real life sometimes wasn't so unbelievable.