He was training to be the first one up in Gemini when his ear issue grounded him 5 or 6 years. Then had surgery done to correct it and gain active flight status again. Deke actually gave him 13 but was over ruled by the NASA higher ups who were concerned because he had not flown in so long. So Lovell and his crew got bumped up to 13 from 14 and the rest is history.
Have you listened to the clips of 14 when they are trying to dock and capture the LEM? They were having a hard time getting the LEM to attach to the command module for some reason. They took several attempts, some of them hitting pretty hard. So Shepard was wanting to suit up and get outside and see if he could somehow man handle it. Fortunately they were finally able to get a hard dock and capture. Chris Kraft sure respected him.