The main reason why Ric Flair decided to quit WWE (there are still ongoing talks to try and bring him back) is due to a storyline idea. A major name submitted a story to creative involving an angle between Flair and another worker. Flair was happy with the angle and if it happened he would have came off well.
Creative then changed the storyline and Flair thought he would have came off looking weak and totally disagreed with the new direction causing him to pack his bags and leave.
These sorts of things happen often so one would assume this had something to do with an angle leading up to Wrestlemania, rumored to be his final run.
Something similar to this happened earlier in the year when Jerry Lawler submitted an idea for him to feud with King Booker. Creative then took the idea, changed it, and used it as a way to get Triple H over in the end. There are rumors that Triple H may be involved with this new idea that ticked Flair off.