Whats the cause behind having pain in your left knee only? been having it since i was a little tll now, it just comes goes whenever it wants too...

Indeterminate: The knee has ligaments, bone, and cartilege. Exactly which one is a problem can be diffucult to tell. Osgood-Slatters in youth is the most common and you usually outgrow that. It is pain at your tibial tuberosity just below the knee. Patella- femoral syndrome is another common condition and motrin and straight leg raising exercises help the quadriceps get stronger and prevent the pain

Prob prior injury: You were probably very active & injured a ligament or tore cartilage in your left knee when you were younger. Now, certain factors coincide to make it hurt: overuse, fatigue, certain activities, mild re-injury. It might worsen as you age, and cross-training will be your best bet: you can run (a bit), but also bike, walk, swim, & in general avoid activities that involve repetition and high impact