Alas, these are questions without answers, because groupthink is not a thing, it does not have an essence, it does not have a definition with necessary and sufficient conditions. It is just a word, and a rather typical one: you see the word used in a few situations, then you form a mental model of when it's most appropriate, and later you use the word yourself.
My mental model of "groupthink" is "generic insult used against large groups that uniformly disagree with me." And as a generic insult it's fine, but it seems void of any real explanatory power.
Given its common use as an insult, we know what groupthink looks like from the outside. However, if we wanted to actually "solve" the problem of groupthink, we'd try to appeal to the people with the most power to change it, i.e. the insiders. So we need a model of what groupthink looks like from the inside. If there is no way to identify it from the inside, then it hardly seems like we can hold people morally responsible for what they cannot identify.
To the extent that groupthink is a thing (which it isn't), we can offer various theories about why it is a thing:
- People who are part of a group mostly associate with each other. There's a cognitive bias to agree with views that we perceive as common.
- We get exposed to our group's ideas more often and with more favorable framing.
- We like the people in our group, and we're more inclined to agree with people we like.
- People who disagree with the group on a particular issue are less likely to voice their opinion.
- People who disagree with the group leave the group or aren't part of it in the first place.
On the other hand, these theories do not offer very compelling solutions. Just because I have some biases does not mean I'm wrong about any particular issue. There isn't necessarily anything wrong with a group selecting for particular beliefs. And though we can partially reduce biases by transforming the way we socialize, it's not clear that this is a good tradeoff.
I offer a concrete and feasible solution. If you're part of a movement, a community, or some other kind of group, obviously that's something that you like. So join a second one. Take careful note of the different norms and worldviews in the groups, and acknowledge the dissonances.