How to be with and stay with someone

Why do some people inevitably drift? They've been through so much together and one day it's like you guys don't even know each other. What happened to acknowlegement at least? I can't connect with her anymore. There is no more waiting for each other, she can't even spare any seconds. Who is that other person? What does she have that I don't? Does history have no meaning? How do you make things STAY?