How many times have you thought you have met the perfect friend or companion only to discover later that they were not what you thought? Their ideals, you discovered, did not mesh with yours? In fact that their ideals clashed with yours?
The problem you might have experienced could be due to your own hastiness. You might have well fallen prey to assumption.
So, what may have happened is that you saw a few traits that complemented your own. From this first vision you extrapolated the rest. They became the perfect friend. Your assumptions clearly were manufactured and existed only in your head but they were there as if manifested by the person perceived. Then… contradictions to those assumptions began to arise. You stepped back startled. You might even have voiced the words “I thought you believe this about ________”. Or you might have said “I thought you liked this kind of food, car, people, behavior”.
Having already decided how they think it becomes a shock to discover they do not. The blame for your assumptions is yours alone, but it may not be entirely your fault. Every person that lives shows you a façade, that which they want you to see. The only way to see through this façade is observation over time. Sometimes, though, it may occur that a particularly well-maintained façade will never be seen through. You may never perceive the true person behind the scenes.
After many years of close friendship it may be discovered that you never knew them at all.