A common misconception in all relationships, is assuming that this new relationship is like one you had before because of random similarities. We use this action as a learning technique. When we see something, we immediately compare it to something else in order to learn. But this only works when things stay constant. As humans we are CONSTANTLY changing. We may be able to categorize someone in our brain by comparing them to someone we once knew, but their placement must be made temporary, or else we are only setting ourselves up for disappointment.
Let me give an example for those who may not be following. Sally just got a new job at McDonald's. This is the second job she has ever had. Before she worked at Burger King. She was fired from Burger King because she was rude with customers, didn't do what was required of her, and showed little interest in her work. She adapted this behavior because the manager was rude, belittled her efforts, and assumed she should know how to do things that she didn't know how to do. When she started at McDonald's, she worked the same way she did at Burger King because she assumed all fast food managers are the same. The McDonald's manager soon became frustrated with her lack of work ethic, and was very rude to her. She soon quit and vowed never to work at another fast food restaurant because "all the managers are the same".
If you are thinking self-fulfilling prophecy right now, you have grasped a key point in my semi-lecture. Assumption is vital for defense mechanisms, but unnecessary when learning about someone. If you are starting a new friendship, or a new relationship, assuming that because they do similar mannerisms in comparison to someone else is unfair to your new relationship. You've set expectations, most likely, without the intent of allowing them to move. And yet as humans, we are constantly moving and growing.
In terms of math, x+y=z. But x+w doesn't equal z. "w" and "y" are different. They may have certain similarities. They may both represent a prime number, or they both represent an even 4 digit number, but they cannot be equal because they are not the same. Assuming "w" and "y" are the same, would therefore make no sense and be ignorant of the idea that x+w=q. (And what if q cures cancer...think about it.)
Let's recap shall we? Comparing people with no room for change is setting yourself up for disappointment. Assumptions are ignorant of new discoveries. You will never learn, if you don't take a chance.