When people get together, live together, or communicate in a tight group with one another, they build up a share stockpile of the same stuff that their personal narratives are made of. A tightly-bound group builds a bank of shared experience. They share language. They have inside jokes, references, and the like...
[Pic is just some random campers, snagged off a search]
There's a reason that small-group "getaways" are used for team-building. It's tempting to say that it's even related to the origins of the Honeymoon. Time together and away from others pushes people into shared symbol-sets. We're naturally really good at small-group bonds like this.
Getting an equivalent bond on the larger scale isn't as easy. There's no consistent "American Experience" from which the people of that nation could draw shared bits of their narrative. Where there is stuff that is naturally spread on the large scale in a long-lasting way, it's almost unconscious - that's the stuff that makes up normality.
Often, people are impelled to try and share narrative stuff across a large group much faster. So, pieces that condense this stuff are created - whole complexes of symbols.
[Pic is Rosie the Riveter. Please don't tell me you've never heard of her.]
In addition to images, large-group efforts also include slogans, ideological systems, role labels for people, and more. More recently, these efforts have included narratives of their own - and this is narrative with another definition. Large-group, media, and political narratives are not quite personal, streaming narrative, and they're not quite story. They're the condensed stuff of personal narratives, built to be absorbed into personal narratives.
This does mean that all attempts at creating large-group blocks of shared narrative stuff, including news programming, textbooks, and common-sense rules, are also attempts to change the way you think. It also means, to the potential annoyance of those sharing that stuff, that they're handing you means to say things they never wanted to say.
It also means that people can submerge into those large-group quasi-narratives to some extent, making them real. Rhetorical talking points can become real thinking - this happens at both small and large scales, but at the large scale, it can lead to extreme drifts. If you have trouble believing this, you may want to watch some Glenn Beck. Or maybe not; after all, when you engage such stuff, you can easily incorporate it - that's the whole idea.
Finally, this leads to an odd conception that some forms of expression are best handled at a distance, as if they were toxic waste. Don't engage Mein Kampf too closely, you might start thinking like Hitler. And that's true to a limited extent, but remember that all communication has this effect to a greater or lesser degree.
We can't create a sanitary world of ideas, without the negative uses of this kind of influence. Nobody would be allowed to talk. It's a basic process, and has upsides and downsides at every turning.