Head-cannon:

They weren't friends to start with. Katniss never left the safe confines of her home, rarely slept, terrified of what the night would bring. Peeta left his house, in an attempt to forget what had happened to him he would do things he would not have normally done. He tried to ignore it, but still he could see the house one over from his, curtains drawn, garden left unattended and no smoke coming from the chimney. He watched for that smoke, to know that the house's occupant was safe and well.

The Capitol doctor suggested that she might need a friend, somebody to trust. Peeta thought that they used to be friends, until he had been captured and tortured. He could barely remember being tortured, in fact, he couldn't remember at all. It was a scab he didn't want to pick at.

Peeta did try. He tried because he wanted the old Katniss back, the girl on fire. But the Katniss he knew wasn't the one who he saw now. The Katniss he saw now was a burnt out shell of who she'd once been. Afraid of sleep, of the dark, of roses. Where had the Katniss who would hunt in the middle of the night gone? Where was the Katniss who would answer her doorbell without fear? Gone, her fire had been put out.

But he tried, he tried in vain. For he wanted to fix the broken person who had once saved his life. He loved her and hated to see her like this. He loved her. Peeta considered small things big victories. Simple things, when Katniss left her permanent residence of the sofa take a shower or sit with him in the kitchen, open her eyes and focus on him when he was talking with her. Not pull away when he held her hand.

Slowly but surely, Katniss began to get better. She began to answer some of Peeta's questions, move from the sofa more often and she even attempted to climb the stairs. She couldn't make it though, she had grown so weak that she could barely move unaided. The biggest achievement of all was that she smiled, she smiled at Peeta. He considered himself blessed that he had gotten one of her smiles, the first in a very long time.

She would still not leave her house, too scared by the outside world. Katniss would sit by the window and watch the Primroses for which her beautiful sister was named, climb the wall outside. How free they looked. Not even Peeta's incessant pleading could tempt her anywhere near the front door.

It soon became routine, Peeta would come in the early morning and again at twilight. One day though, Katniss did not answer the door. Peeta grew worried and finally managed to break the door down, the fear giving him strength. He found Katniss lying on the sofa, eyes closed. He smiled, she was finally sleeping but then he noticed the bluish tinge to her lips, the unnatural cool of her hands. It was then he noticed the letter addressed to him, folded into her palm.

Dear Peeta,

I love you, I love you more than there are words or stars, seconds gone or seconds to come. But I can't live with the fear. The fear that if I show I care for someone, life will turn around and take them away from me. It happened with my father, with Rue and my baby sister, Prim. I'm afraid that if I show I care for you, I'll lose you too and I won't be strong enough to cope. I never was. So I thought if I removed myself from your life then nothing will happen to you, I'll have left of my own free will and will not be able to get close to anyone, therefore eliminating the problem. I love you, I have, always. I love you real or nor real? Real.

All my love

Katniss

Peeta folded the note back up and placed it on her stomach. He then saw the berry, that damn berry that had caused all of this in the first place. He then saw there was one left. Peeta held his tears and gently swept the berry out of Katniss' palm and into his own. He brushed her hair out of her eyes and whispered 'I love you',' before placing the berry in his mouth. He lay down beside Katniss and took her hand in his own and swallowed the berry.

It was tragic, they said, two young people battered and bruised by war, so traumatised that they only way to end their pain was to take the plant that started it. The two watching down from the sky knew better though. Peeta knew he couldn't survive without her, she was the whole reason he was alive in the first place. Katniss knew she couldn't lose him, she had lost so much in her life that anything else would have pushed her over the edge, unfortunately she was already there. Neither of them wanting to live, and face life's struggles and hurdles, without the other.