Doctor Who, 4x13
When you consider the Doctor and Donna’s relationship — how they each brought out the best in each other throughout the season — this was actually the perfect thing to happen. Their yin and yang became literal when they each got the part of each other they were missing.
And of course, Donna couldn’t stay that way, because the Doctor is like the sun and his companions are all Icarus — get too close, and your wings melt and you drown. (Hence why Martha is arguably the “best” New Who companion for leaving.)
Meta-crisis Doctor got the humanity he’s always missing, always searching for vicariously through companions, but at the price of his alienness and his immortality. He could be human and stay with Rose, but he couldn’t last forever.
Because only the Doctor lasts forever.