Seems we assume our friends agree with us, just because they are our friends.  Yes you agree with your friends more than with random folk, but you agree less than you think:

Friends disagree more than they think they do. In particular, friends are typically unaware of their disagreements, even when they say they discuss the topic, suggesting that discussion is not the primary means by which friends infer each other’s views on particular issues. Rather, it appears that respondents infer opinions in part by relying on stereotypes of their friends and in part by projecting their own views. (more)