Looking back...while going forward

They all say that you should never look back while you are on a road going forward. My question is why? I am happy where I am, and sometimes happy is not even a fraction of how I feel. I love Lisa. She is the greatest thing that has ever happen to me, my savior my everything. Now I cant say that there have not been days where we have both questioned why we even tried. And we both have been with one foot out the door ready to leave, but even with both feet out our heart never followed in our footsteps. We have both made our mistakes in this some that I will never forget and some that were seemed impossible to forgive, but we both knew that the biggest regret would be letting it all go. I still look back and I keep a rear view mirror of my life in my pocket not to trance in memories but to remember where i never want to be again. Not to say before Lisa everything was horrible, it wasn't good, but this right NOW is so much better.
Now hopefully we will be starting a NEW journey together creating a family between us.