Originally Posted by Listentome:
“the Master is definitely alive. I think he insisted on dying o give the Doctor the false impression that he was wrong about him. The Doctor said the Master would never let himself be killed, then TM allows himself to 'die' in the doc's arms.
One thing I like about DT's Doctor is his gullability. He believes things. He has been proven wrong over and over about things he thought were certain ie: no more Timelords, the Racnos all dead, can't move between universes etc...so now he has almost become accustomed to being proven wrong, he accepts he was wrong about TM so easily, without question that he has been double-crossed. the Doctor should know better really, but has allowed his emotions to take control.”
I think you're right about Ten's tendency to believe things he shouldn't. I think it's because, at this point (end of series 3), he's just so tired and empty. He's been alone for so long and everything has been taken from him. He's lost so much and I think that the reason he can believe the Master is dead is because, now, he half-expects to lose. He knows that everything he wants and cares about is taken from him; why should the Master be any different? He had that period of hope, when he was not alone and he allowed himself to be seduced into believing he could spend the rest of eternity with the Master, but when that was taken away from him, he had no difficulty believing it because I think he just thought "why not? why shouldn't I lose this too?"
I think the Ninth Doctor would have doubted it, would have looked for the Master's Plan B, but Ten just expects loss and accepts it when it occurs. He's just so broken at this point.