Even though it must be extremely daunting to take on, the role of the Doctor is right up there with the most sought-after parts in the TV business.
With such a huge legacy and a devoted global fanbase, the level of scrutiny these actors are forced to endure is truly extraordinary, and a string of bad episodes could doom them to be remembered in a negative light for the rest of their career.
With that in mind, it's not surprising that most actors only play the character for a couple of years. It must be an honour to step into those shoes, absolutely, but the amount of pressure hoisted upon their soldiers is an unreasonable amount for someone to carry for five or more years, and because the show is so highly publicized, we often hear stories about what prompted the stars to leave Doctor Who behind.
Some actors have straightforward motives, others were victims of unfortunate circumstances, and a few didn't have the happiest of endings. Whatever the cause, it's a subject that each leading man has touched on at one point or another.