I read somewhere it was due to frustration at not getting a shot at Ali. In his book Foreman says he had a 'vision' after his loss to Jimmy Young,in the dressing room afterwards....although from interpreting what happened it sounded more like an 'anxiety attack'. Some say he went to work for the church...? I think that is as believable as him saying his comeback was to fund the church... Given that he was around 28 and should've been at his peak...why do you think he 'retired'.?
Losing to Young was a huge setback. No fighter - particularly someone who's already been there - wants to start all over again and work their way up to the top in the mere hope that they'll get a title shot. It's a very tedious, exhausting - both physically and mentally - process.
I think that is the difference between the all time greats and the greats. The All time greats actually do want to go through the exhausting physical and mental process again. That is why Muhammed Ali s Losses to Frazier and more importantly Norton, and even Holmes didnt stop him from wanting to climb the ladder. That is a huge part of what made him the greatest.
Ali unfairly made George wait excessively for a rematch. George was already the SOLID number 1 contender, and shouldn't have even had to face Young for a shot. Young was very much George's nightmare opponent, and the rest is history.