He knows what he wants, he always has. He can't remember a time when he didn't want her. Sure, the way in which he wanted her has changed over time but it's always been her. When he was five he was in awe and wanted to hear her sing forever. When he was twelve he was scared and just wanted to save her, feed her, protect her from the world. When he was seventeen again he wanted to save her but he also wanted to hold her, kiss her, touch her. And now after everything that they have been through together in the past two years (the games, the Quell, his hijacking and the war) he just wants to be with her, share the new normal, the every day. But she has to want him too.
So far she hasn't shown signs of wanting anything. She wouldn't eat if there weren't people around to feed her. She doesn't bathe or leave the house. She doesn't talk. She sits in her kitchen. He can't tell if she's dwelling on the past and all that's been lost. His instinct says yes, that is what's dragging her down but she seems lost even to coherent thought.
He knows that she still has the nightmares though. Some nights, when he's left his window open to let in the breeze, he can hear her scream. It's the nights where he doesn't hear anything that he worries the most because on those nights he worries that she's given up the will to live entirely and that's his greatest fear.
As long as she is still breathing Peeta can maintain hope that she will come back to him. Hope is all he has left. Hope is what gets him out of bed in the morning and into town to help rebuild. Hope is what makes the nightmares, because he has them too, bearable. He might have given up hope long ago but Peeta doesn't know how to live without wanting Katniss. He's wanted her all of his life and now he doesn't want his life without her.
So he plants primroses in her yard and he brings her cheesy buns and he waits for the day when Katniss will want him the way that he wants her.