River Song

"My past is his future. We're traveling in opposite directions. Every time we meet, I know him more, he knows me less. I live for the days when I see him, but I know that every time that I do he'll be one step further away. The day is coming when I'll look into that man's eyes, my Doctor, and he won't have the faintest idea who I am. And I think it's going to kill me."

 (Doctor Who - The Impossible Astronaut)

River: I can't let you die!

The Doctor: I have to die!

River: Shut up! I can't let you die without knowing you are loved — by so many and so much — and by no one more than me.

The Doctor: River, you and I, we know what this means: we are ground zero of an explosion that will engulf all reality. Billions on billions will suffer and die.

River: I'll suffer if I have to kill you.

The Doctor: More than every living thing in the universe?

River: Yes.

(Doctor Who - Ep.  The Wedding od River Song)