Could you tell us how old you are?. Because based upon the information you have given, it seems an age related common orthopedic problem of osteoarthritis of knees. In simple words, it is the result of age related wear-tear plus drying up of the lubrication inside the knee joint, accompanied by weakening of ligament supports around the knee joint (leading to narrowing of the joint space). It could have started unilaterally, left knee in your case, due to some more precipitating factor on that side ( e.g. any kind of trauma to your left joint, hastening the progression of already existing bilateral osteoarthritis, earlier in your left joint), but mostly it affects both joints equally. If you are young individual, then rheumatic arthritis and some other arthritic diagnoses could be there, based upon the physical examination and some baseline investigation work up.