No "euphoric" side effects when on pain killers?

I don't seem to experience the "pleasant" effects of pain killers. I have been on Vicodin after having my wisdom teeth removed, and after spraining my back, I was prescribed both Norflex and Tramadol. However, I have never experienced any sort of euphoria or "high" from these medications. The Vicodin and Tramadol do work (mostly) at relieving the pain, though the muscle relaxer seems to only slightly help. I was just curious as to the reason why I am not affected the same. I am not strongly resistant to over the counter pain relievers such as Ibuprofen or Motrin, and both are effective and aiding in common aches and pains.