Katniss is a bird, strong and fast, but timid and frightened by the slightest movement in her direction.
Gale is his namesake. He is the wind that pushes her forward, challenges her to go faster, keeps her in the air when she begins to tire. She has no reason to fear the wind. The wind pushes her, guides her, supports her.
Peeta is a patiently waiting tree. He is strong and sturdy and all too willing to let the beautiful bird rest on his branches if she feels so inclined. Even if it means his branches and leaves will be torn apart by the harsh wind that will forever be at her back. But Katniss is too scared of the tree and its hopeful green leaves that shudder when she passes, so she continues to fly no matter how much her bones ache and her body cries out for just a moment's rest.
Peeta watches her flirting around effortlessly in the sky and hopes that one day she will tire of her fearful flying and take a rest in his open branches and allow him to comfort her in a way he knows she would never admit she needs, but he knows she will only stop flying, stop running, when she has to. When her wings are broken and the pain becomes too much, only then will she take refuge in his shaky leaves and solid branches.