Is flap surgery with bone graft required for bone loss alone? How long will it take to form new bone?

I had a flap surgery recently with a bone graft to my lower teeth. My doc said i got 40% bone loss after seeing my OPG. I had no symptoms like swelling or inflammation like in periodontal disease? I wonder how the bone loss occured without symptoms. I had braces 10years ago after pulling 4 teeth up and down. Can i know what caused bone loss in my case? Can periodontal disease be painless?