I have always wondered about this. It used to be said that ,the first impression is the last impression..
In my view when you meet the person for the first time..he/she will always put forth their best aspect of themselves..will try to portray a perfect image. I'm not sure its deliberate though..it just comes naturally maybe. In this case you are super impressed and love the person you've just met..
Now on the flip side..What if someone has an extremely bad day or going through something serious in his/her life..now this person you meet for the first time maybe you will not be that impressed and happy about..So we are kind of not giving this person a second chance as we have built an impression/image of this person in our minds...
So in my opinion the first impression is Not always the best/last impression...this is what I feel..looking forward to your views.