Ooo! I had another thought today (probably one that several million other people have already had – I’m a bit slow!)
OK, here it is: two Time Lords equals possible pathway between universes (is this right?), so, Doctor realises this, but of course, he has to forsake the chance of an emotional reunion with the one he loves, in order to save the multiverse by destroying the Master. Heartache all over again for our hero, as not only has he had to destroy the only other Time Lord (that he knows about, at least), but once again loses his chance to be with Rose (someone pass the hankies…)