In my case, far too many rock concerts and firearms.

I attribute the major portion of my loss to about a year as a riveter in an airplane factory long before hearing protection was required or even suggested.  I also spent more than two years on a twin jet night fighter as the airborne intercept/radar operator with the constant whining of the engines in the cockpit.  Firearms and artillery, rockets, and mortars during line duty in Korea also contributed to my loss. 

Shortly after my Marine Corps service at about age 25 I could no longer hear well enough in my left ear to use the telephone on that side (I'm left handed so it was a significant loss) and switched to my right ear.  In the past 30 years in particular my hearing has taken a nose dive.  I rarely use my hearing aids which don't help a lot anyway and also use headphone connections to both my stereo and TV.