Falling in love for the first time involves unbridled emotions, raging hormones, butterflies in the stomach, fairytale beliefs about the future of ever after together. You've never been hurt or had your heart broken. Therefore you don't hold anything back emotionally.
After you've had your first breakup or that first love is out of your life you can't help but become a little wiser simply knowing not everything works out the way you hope it will. Once you've gone through an experience you can never truly recapture the complete "newness" you felt the first time around. Eventually the "unexpected" becomes the expected. You become more guarded emotionally.
If you've had multiple relationships over your life span you recognize there is an "infatuation phase" with every (new) relationship. Your first love though was not seen in your eyes as being a "normal" part of falling in love because you had never done it before!
We tend to remember our "first" of everything we consider an important phase of our life: first job, first car, first apartment, first time we kissed, and the first time we had sex. No matter how many jobs, cars, apartments, kisses, or times we have had sex we will always remember (the first) whether it was a good or bad experience.