São Paulo felt odd to Dr Brent Coker when he visited, but he was surprised when he worked out why.

Outdoor advertising is illegal in the Brazilian city, so his brain – so conditioned to taking in marketing messages – had decided that the ad-free environment was unnatural.

For Dr Coker, who teaches Internet Marketing and Social Media at the University of Melbourne's Faculty of Business and Economics, it was yet another sign of how dominant advertising is in our lives.

We pick up our smartphones first thing each day, checking our emails, our Facebook feeds, and apps such as Zite.

In those few minutes, we are bombarded with advertisements, not all of them overt. Those disguised as "user content", such as the comment from a friend on a post by their yoga studio, which includes a photo of the space and a link to their website, also count.

Most of us accept advertising is part of life, whether on billboards, on TV, or our Instagram feeds.

And as the advertising space becomes increasingly cluttered, the challenge for marketers is finding a way to get their messages noticed amid the noise of everyone else's.

Indeed, on social media, it's no longer enough to just get noticed. Every marketer wants their content to go viral – to be circulated widely and rapidly across social networks. It's not easy to make that happen, as many marketers have discovered.

But according to Dr Coker, who covers the subject in his forthcoming book, Going Viral, there are some key reasons why some content will never be shared widely online.

1. It's controversial

Dr Coker says many marketers and social media users tap into the controversy of a topic to generate attention and interest, but it's no guarantee of viral success.

"The antithesis to viral marketing is to create controversy," he says. "The theory is that if people are shocked, then they'll at least take notice. But there's a fundamental problem with using controversy to get noticed.

"Creating controversy relies on creating an emotional response using negative emotions such as disgust, grief, or occasionally fear. These are not pleasant emotions, and generally people will try to avoid feeling them."

2. It doesn't have social currency

Contrary to popular belief, sex doesn't sell. In fact, raunchy content can actually work against you.

Dr Coker says: "Most people would feel quite awkward sharing something sexually explicit in their social networks, since people don't necessarily know all their connections that well.

People care about social norms, and people choose their actions based on what society expects.

What he is explaining is our desire to earn what he terms social currency. We want to replicate positive emotions in our communities, because we believe it adds value to our own image.

"One way we earn social currency is by contributing in a positive way to a group. Shared interests are one of the forces that bind a group together," he says.

Common sense tells us that because most people do not wish to make others feel bad, negative emotions should be avoided. Content of a sexual nature might capture people's attention, but often also creates a sense of awkwardness.

3. It doesn't fit your tribe

Hipsters have big beards, Harajuku youths exaggerate street fashion, and bikies love showing off their Harleys. It's all because as humans, we are tribal in nature, and feel the need to be part of a social group.

"People's desire to express who they are, and signal to others something unique about their choice of group membership is called membership cues," says Coker.

Fashion and attitude are signals to others within the group we identify with. We decide if we would like to belong to a certain group based on whether we agree with, and want to promote, the ideals and values of the current members.

If your campaign has no way of allowing people to signal to others something about theirideals and values, it will not be shared.

4. It doesn't invoke (positive) emotions

If you hear a good joke, or read a funny post online, you'll want to share it with your friends if you think they will like it, and will laugh.

In other words, if we expect the emotion from the other person to be as positive as (or even more than) ours, we'll share. But conversely, if we're not sure how they'll react, we won't.

We do this because it makes us feel appreciated when someone else feels grateful for having been able to enjoy something shared with them.

5. It underestimates anticipation

Life is full of surprises, and most of the time, we have no idea what the future holds. This uncertainty creates a sense of anticipation, whether it is positive – looking forward to a holiday, for example – or not, such as waiting for test results from a doctor.

"Anticipation is an important part of the story of life, and for that reason it's often found in movies and advertisements that have gone viral," says Dr Coker.

"The reason why anticipation causes you to share is because it affects you physically as well as mentally."

We tend to be more sociable when we are excited, physically and mentally alert.

We feel a desire to share the information that has made us feel this way, because of the spike of adrenalin that is coursing through us.

It doesn't actually matter if the content isn't entirely positive. People often share content that generates shock and fear. This is because such emotions are so strong that they tend to trigger a physiological response, leading to physical changes (increased heart rate) and mental alertness (wakefulness), similar to the sense of anticipation.

This mental alertness is the opposite of boredom, and so engaging and compelling that the person involved in the event will feel the need to tell others about their experience.

6. It plays (too) fair

In life, it's often pragmatic to be diplomatic when it comes to voicing your opinion about something, or to simply say nothing at all.

But sometimes on social media an opinion just really wants to be heard. How often have we seen the same post (or the same topic) shared multiple times by different friends?

Think of Cecil the Lion. Hunting is by no means illegal in many parts of the world, and certainly not where Cecil reigned. However, when a beloved animal known to so many was slain, the online discussion wasn't simply about his death. The debate that was sparked wasn't even between the pro- and anti-hunting camps.

Almost all of the comments posted stemmed from a demanded sense of justice from those who were outraged that rules had not been followed, that negligence was prevalent, and that an innocent, magnificent animal died. More than that, it had died slowly and painfully, simply for the pleasure of one man who had had an unfair advantage over a defenceless lion.

Dr Coker says: "An unfair disadvantage creates a sense of injustice, whereas a simple disadvantage will tend to be ignored.

Creating a disadvantage without unfairness might even have the opposite effect of what is intended, and create a sense of schadenfreude.

If your content does not cause your followers to experience a strong sense of empathy arising from an injustice, it will be difficult to get them to share.

Ultimately, creating shareable content needs an understanding of human behaviour, emotions and expectations – and the knowledge that not all publicity is good publicity.