It also takes over the actor's life, pretty much, whilst they're in the role. One of the reasons Tennant's last year as the Doctor had some specials rather than a full series, was so that he could go and play Hamlet at the RSC, along with another play and a tv drama.
Most of the Doctors only did a few years. Tom Baker was very much the exception to that. Pertwee ( I think ... might have been Troughton) gave Davison the advice not to do more than three series; and then Davison passed that advice on to Tennant :P Any longer than that and there's a serious risk of screwing up your future career, I suspect. Tom Baker spent seven years (iirc) in the role and had huge difficulty finding a place for himself afterwards.
Eccleston was a slightly different situation. he wasn't particularly familiar with the programme (heard of it, obviously, but don't think he'd every really watched it) and they knew when they were making the series that he would only do one run. He was just treating it as an intriguing role with good scripts. They had no idea at that stage if the nu-Who would even succeed.
I'm kind of pleased by the length of Tennant's run as the Doctor. RTD put his Doc through the mill and then some. We saw him cope with loss, then love, then loss again. With the memory of his people's destruction, and his own role in that. Being alone, then finding the Master, his one time childhood friend and arch enemy; try to help him, then have him die in his arms. He's lost every person he's ever got close to, up to and including his best friend, who will never remember he exists.
Kind of hard to see, dramatically, where they could take him now. He's run the emotional gamut.
Add to that the fact that RTD and a couple of other key peeps all left at the end of the specials, and it seemed right (according to interviews I've seen) that he leave at the same time. Tennant was tempted briefly to change his mind and stay on for Moffat; but as he said, if he hadn;t left now, he;d have ended up staying until people made him leave :P Better to leave when people still love you.
It's the right time for a change
