River Song had long ago made peace that her life would not be easy nor pleasant. She had made peace with the fact (or at least she tried to) that she would always lose the ones she loved.

First it had been her father, vanishing into thin air while she was looking the other way, chatting mindlessly about things that didn't matter. She lost her father in the blink of an eye. No goodbye for her.

Then it had come her mother. Salty tears, a good bye, and a mission. "You be a good girl, and look out for him." While she didn't know if she could accomplish the first part, she knew she would try her damnedest to do the second. For her mother, for herself, and for everyone else who needed him to be okay.
Then she lost him. After her parents died, they drifted apart. They tried, they really did, they tried for 105 years. But she was right in saying that only one psychopath per TARDIS was best. She still loved him and she still loved her. But, sometime it's hard to love someone when every time you look at them you see yourself in a different skin.
They fight, they run, they make up again. River and her doctor in an endless wheel.

Then came the singing towers. He gave her his screwdriver. And he cried.
She held his head and ran her fingers through his hair. She doesn't know then why he's crying, but it doesn't really matter.
That night she heard yelling. So she crept from their bedroom where her husband was not. And found him yelling at the TARDIS. Kicking the consul and bashing his fists into the controls. She pulls him away before he can hurt himself and tells him that everything will be okay. Nether of them believe. One because he knows the future, the other because she knows her Doctor.

A week later she's running through the library and she knows now why he cried.
This man looks at her with the eyes of a stranger. He's so angry this doctor she sees. So much fire just under the surface trying to break through. Her man, her doctor hides his fire far better and she knows why. It makes her sad to think that in order to make her love, her Doctor, this man must be beaten and broken and burned by everyone and everything more times than she can count.

She chooses to die for him. Of course she does. It never was a chose to begin with. His life is worth so much more than hers. And she would lay down her life a thousand times over for him. Her love. And while she has her own selfish reasons for making sure he lives, she also realizes, well hopes really, that if he truly loves her as she thinks he does and as she loves him, he wouldn't concede with a few shed tears and and a final gift.

So she takes a leap of fate like her parents did so many times before and so very long ago that her love will be there to catch her.

And he is. Because as long as time still stand, against all odds they will be there. A perpetual fixture in creation itself, River and her Doctor.