Why do we do this? Probably because it is comfortable. You have to think a little bit less when everyone else thinks the same way that you do. You're not challenged to critically think about your position.
It is interesting because we assume that everyone had or should have had the same experience growing up that we did. If they didn't then there must be something wrong, they must be a little strange. I know you don't actually think that, but the subconscious seems to and it influences what you do.
In my church body one of our favorite things to argue about is the style of worship on Sunday morning. Should it follow a traditional format or should it be more contemporary? (Note: Just to be clear -- This post isn't about the merits of either position in that discussion)
One thing that I've noticed in these conversations is that people who participate in them often assume that most people grew up with their style and that, for that reason, should consider it to be the norm. Because they've only experienced one type of service their whole life, they seem unable to find value in any different style. This makes the conversations challenging because the discussion is no longer based on the merit of the content or the style or what it communicates, but rather it devolves into an issue of personal preference. We want people who are just like us rather than discussing the deeper theological points about what worship is and how it is faithfully practiced.
I think that having a diverse spectrum of friends is great. My Facebook newsfeed is incredibly interesting. It ranges from the far left to the far right and includes many moderate folks too. This gives me many different perspectives on a wide range of topics and helps me to understand the various opinions and struggles that people have with different issues.
With all of these different perspectives, critical thinking becomes very important to help process through all the positions. Devoting one's self to is critical thinking is a challenging but necessary part of life. When we choose to no longer critically think about things, it becomes very easy to get carried away by the current of whatever is popular. This current will drag you along with the crowd and drop you off wherever you feel most comfortable -- usually around the people who are just like you.
Critical thinking is important to me and I will write more about it later. Right now I'd encourage you to find a friend who has a different position on a mutually important topic and take some time to understand your friend's position. You don't have to agree with it, but in the process of understanding it, you will not only better understand them but you will also better understand yourself.