First Impression is usually the most deceiving one

I really don't understand why it is said that first impression is the best impression.

On & again in my life it has been proved that first impression is usually the most deceiving one.

The otherday, i had a pretty long conversation with a personwhom i hated in the beginning but then felt that i made a BIG mistake. Only during during the conversation did i discover that i had greatly mis-understood that.

As a first impression, i though the person to be someone whom i would define as a true JERK. But actually, it turned out that the person is someone who is matured, understanding, witty, humorous and almost everything that needs to make a person comfortable in a long chat. But, underneath all these things there is a very high feeling of Insecurity which make this person act in all these weird ways.

This is not the first time i was in a situation like this. So the only thing i have understood is that, never judge a person by his/her looks. If you really want to judge a person, then get close to that person.