See the short introduction to these essays and the rest of my sociomedical essays here.
The word "mantra" comes from two Sanskrit words, man (mind) and tra (tool, instrument), which together mean something like "instrument of mind". It commonly refers to a verbal string chanted during meditation. A mantra is not so much a prayer, but something to distance us from the earthly world, something to help us stop thinking. In some religions it is thought that chanting a mantra makes it true.
Meditation is a healthy practice which can be recommended to almost everyone. Letting go of thinking may be beneficial in certain amounts, but for many people it is the norm. Too many people talk of opinions when they really mean personal mantras. In many cases it is erroneous to talk about "forming an opinion". It is even questionable whether it is accurate to even talk of an opinion. Commonly the so called opinions are not formed, people just inherit them from their parents, relatives, teachers or friends.
One of the most popular sources of mantra opinions is spiritual fiction, specifically a subgenre known as religious manuscripts or "holy books". One could also view them as self-help guides from a time when publishing editors sadly did not exist. Not many people would follow the advice of a 500-year old self-help guide, yet strangely many people are willing to adapt mantra opinions from a religious self-help manual far older than that.
But spiritual fiction is not the only written source of mantra opinions, unless you want to include the media under that umbrella as well - which may not always be so far-fetched. Some people really do believe everything they read or see on TV. In case of a controversy people will just stick with the side they were originally on. Studies show that after choosing their side in an argument, the vast majority of people will ignore all contrary ideas. In some cases factual evidence showing their conceptions to be flawed may only make them even more convinced of their own view.
Authority figures like teachers and doctors are another source of mantras. Their job, of course, is to be authorities, but many people interpret that as infallibility. Sometimes when people are presented with information which contradicts their view, they will insist on "my teacher told me!" and refuse to acknowledge any conflicting facts. As adults, many people fortunately do realize that their own parents are infallible and that many of their ideas may be outdated, yet there are also people who even as grown-ups consider everything ever said by their parents to be true, whether it is "carrots make you see better" to more harmful ideas, such as particular ethnic/religious/political groups possessing certain properties.
Mantra opinions may even originate from the advertisements people see, much more often than one might expect. It is hardly strange when you consider that advertisements feed us mantras in the form of slogans. People love slogans, people love quoting things and compressing complex things into a short string of words they can recite, whether it is "men are from Mars and women from Venus" or "communism is a nice idea, but can never work in practice". In a way a mantra is also like a brand.
A mantra should be sufficiently abstract so that it cannot be proven wrong. Even if it has a somewhat factual or rational background, it may serve to skew or misinterpret things. Some mantras do have a historic basis behind them, but that has either become obsolete or since proven to be false. Even mantras that can generally be considered as facts are often not perfectly accurate, because they are generalizations. There is nothing wrong with generalizations if they are treated as such, but often they are not.
You could take the side that all kinds of scientific information is hazy and bound to change. Some people choose this side and turn it into a kind of nihilism. Obviously opinions are different from facts. Thus you can just as well form an opinion based on beliefs, prejudices and whatever your environment happens to feed you. Because the way mantras explain things they can even be considered a form of religious practice. When we cannot or do not want to trust science, we let mantras guide us. It is a bit like an inbuilt horoscope or compass.
All of us have some personal views that we picked up somewhere along the way and are unwilling to give up, but there are also common mantra opinions shared by a majority or a large part of people in a given society, either negative or positive opinions. They often have no rational basis at all, but that does not stop people from using them as arguments. Human communication is a complicated sport, but mantras bind us together and make it easier for us to understand each other.
When someone disagrees with their mantra opinion, people tend to get confused, even angry. Their programming goes off. Since they often do not have any arguments to back up their mantra, they resort to personal attacks. Sometimes people go as far as claiming the other opinion is "invalid" or the opponent has no right to it, because it differs from the common mantra, which serves as an important part of the person's identity. Often it is a case of "we believe what we want to believe". If this does not sound like it applies to you, put it in the reverse. Would you believe something you did not want to believe?