Good question. Here's one possibility.
The Face of Boa was millions of years old. Jack had obviously undergone mutation to become the face of Boa. I think after the time that had passed and the changes that took place, he developed the capability to choose to die. Just like when the Master was dying and the Doctor begged him to regenerate. The Master choose not to regenerate and just let go.
Besides, if rule one is that the Doctor lies, then
when he said Jack could never die, was he sayinng forever?
Do you watch Torchwood with Captain Jack? Did you watch this season's Torchwood: Miracle Day?
If not, SPOILER ALERT if you plan on watching the series............
Mekhi Phifer joined the show as Rex Matheson, a black CIA "golden boy" operative, who teams up with Jack and Gwen. At the end of the series, for reasons I won't go into, he has had all his blood drained out and replaced with Jack's. When he gets shot, he heals immediately. Raises the question on if he is actually the face of Bo instead of Jack. If you remember, the face of Bo was dark complected and Jack is as pasty white as a vampire.
posted over a year ago
He can't be that guy from miracle day as he doesn't know the doctor, the face of boe clearly knows about him and the Master, and the face of boe is a massive head! And the first thing you thing of is that he has a different skin colour? Whatever made hi, like that could have changed his skin too you know?
The Face of Boe was several billion years old when he died. After all that time, he didn't have a lot of the energy that kept him alive left. What little he had left was used up keeping the people of New New York alive.
Well I always thought it was the energy from the TARDIS that kept bringing him back. The energy has got to run out evenually. So maybe he can die as many times as he likes but only for a certain length of time? Say until new earth?
