Doctor: I don't age. I regenerate. But humans decay. You wither and you die. Imagine watching that happen to someone who you-- Rose: What, Doctor? Doctor: You can spend the rest of your life with me. But I can't spend the rest of mine with you. I have to live on. Alone. That's the curse of the Time Lords.
Let's all fill in the blanks! What was the Doctor going to say? :P