The most rational thing to think is that youko became a pure blood vampire and had to be killed, based on how future nagisa reacted, but we all know this is won't happen
That's what mostly is with love, you can't maintain the same old friendship till the very end.
You may say it's not true but even the people who do keep their friendship are not the same as they were before.
Note: My reference is towards love triangles between friends.