Consider America's entry into World War II. Ask a modern American, or anyone really, what the justification for entering the war was and they'd probably say to stop Hitler. Asked why they'd care about stopping Hitler, and the answer might be that Hitler killed several million Jews. A pretty good reason.
But Americans in 1941 had no idea about the Holocaust. Yes, there had been reporting on persecution of Jews under the Nazis, and there's still controversy over who knew what, when, and how, but in general they didn't know. So why did the USA still go in? Well, no doubt there were still good reasons. But they become a lot less cut and dried when we take into account what was known at the time. The real reasons at the time were geopolitical. Only later did the moral case become obvious, and retroactively applied.
I sometimes wonder what might have gone differently if the Nazis were not quite so cartoonishly and stupidly evil, confining themselves to "just" taking over Western Europe and not trying their hand at genocide quite so enthusiastically, or if Japan had not been so foolish and arrogant as to attack Pearl Harbour. Would the USA have stirred in this alternate universe, if Hitler had one less screw loose and Tojo had one fewer bottles of sake? If not, history could have easily turned out very differently.
I'm pretty sure this is false and that most people know the US entered WWII because of the Japanese attack on Pearl Harbor.
Obviously though, the opinion precedes the "reasons" for the opinion. IE, they are "rationalising".
In war, this is obvious to us but it applies pretty widely. People's opinions/conclusions are based on identity and group membership. The "reasons" for opinions are applied retroactively in a lawyerly, best-available-agrument fashion.
This probably applies to you too. What you believe about Brexit, border walls, Chinese political reforms, the free market, #metoo, agile... the reasons for your opinions are probably not your reasons, and your social sense probably plays an uncomfortably large role in determining them.