Why are we so divisive?

It is very well known that humans are creatures of habit. And one main habit in societies is to congregate with those that are like us. Its somewhat natural. Whites live around whites. Hispanics live by each other. The Jocks and the cheerleaders make a large divide between themselves and anyone who might no more about a computer than how to check email, sports, IM, and maybe some inappropriate content. We basically ove ourselves so much that we enjoy to be around those that remind us of ourselves. Or, maybe we are just so comfortable that we won't venture out to new things? Yet within this 'love-of-the-like' we hate what can be so much like us yet, not in the way that we like. (if that made sense) We are eternally a breed of creature that holds tight to what is comfortable, and shuns what might challenge us to something better.
Now, I am not saying that it is the greater good to seek out dissonance in life. But, balance my friends is the key. Moving things along. I am predictably intrigued, yet slightly perturbed with a conversation I had today in the midst of slinging the 'Bucks' greatest coffees. I found myself in a very non-heated debate on the idea that Catholicism in not Christianity. Now, we were both in agreement that there definitely are Catholics that are Christians. But on the basis of some vague knowledge of Catholic Marialogy, as it may be called, One of my co-workers was ready to write off Catholicism. Her statement,"But, the pray to Mary, that's not biblical, it's not Christianity" My statement "I understand, would agree to the idea that there are some theological views that, in what I have seen produce some very incorrect practices; I think that you get in to some dangerous territory in making the blanket statement that all of Catholicism is outside of Christianity"
Now, this debate could take many different turns, namely the different views of Biblical interpretation, or the success and practice of assimilating/discipling members of a body to the larger bodies true doctrinal views. And, I don' think that any faith, religion, or organization has really figured that out anyways. But what bothers me and what I would like to bring to light is the divisive nature that is within this debate. There are so many differences and, yes for the sake of assimilation/discipleship, we wouldn't want to confuse newcomers, but why aren't we looking toward unity first. Why can't we seek to form a basis of common belief. Rather than setting apart each and every differing view of faith and trying to hold 'mine' up as the "right" one(and I cringe as I say this)

My heart here, is something close to Philippians 2. "Being in the very nature of God, Christ did not see Equality with God Something to be grasped. He humbled himself for the sake of others." (My own paraphrase) In that spirit we should be seeking to join with others and not draw lines......