You are mis-understanding the meaning of the word 'agreeable'. Agreeable people do not tend to agree with other people, they tend to get along well with other people.
"agreeable: quite enjoyable and pleasurable; pleasant."
"agreeable: quite enjoyable and pleasurable; pleasant."
People in general are scared of being singled out for disapprobation by the group and so they self-censor and don't create new ideas in the first place.
Not necessarily. As long as they are civil and able to articulate their differences in a good way chances are the viewpoints will be shifted and possibly converge.
It's a problem when people become so invested in their viewpoints and opinions that they become a part of their identity. That's one way to become a person who is going to react in a not-so-nice way to seeing those viewpoints and opinions challenged.