They did it every day, and it was something someone raised in a non-magical household never got used to.
It was the first thing you learned. Fire could hurt you, burn you up. He remembered watching Dudley sending letters up the chimney, screaming if it burned instead of flying upwards. He was never allowed to try. Maybe they feared he would, even as a child, be able to enchant the fire and burnt he house down. Or have the presents appear immediately. They always barked at him to stay away whenever he went near it. He was allowed to chop the wood, but not light it. Fire was dangerous. It could hurt you.
He still used it everyday. To get to work, get supplies get home. The Floo network was anonymous. Easy to hide your face and pretend to be protecting yourself from the smoke.
He knew the fire couldn't hurt him, often he could barely feel it. His insides had been frozen since the war. No longer the symbol of hope against an oppressive monster, he was a constant living reminder of those who had been lost, of the violation of the safest places in England. The coldness inside him spread, and his friendships had suffered.
When the others were celebrating, the victory, the downfall of Him, the hopeful new age, his heart had iced over, numbing him to any of the joy surrounding him. He only saw the coldness of his fallen friend's eyes.
It happened on one of his expeditions to get more Floo powder, ironically enough. He was wrapped up against the non-existent cold, against the eyes of other people. The person before him turned to leave and he could feel their gaze raking over him. Pushing forward, he asked for his powder and paid. When he turned to leave those grey eyes were still on him. They left together silently, and ended up in a bar. Neither leading, just falling into step as if it was natural. Draco pushed a firewhisky towards him. It didn't even burn on the way done. It was nothing. Long fingers grasped his wrist.
"Look at me."
Their eyes met. Cold grey eyes that weren't cold at all. Strange that grey could hold that much fire, that much passion. They were the same. But they weren't frozen. They were greedy fire burning up everything they saw.
His head tilted, and they seemed to be having an entire conversation in silence. His wrist felt hot under Draco's grip. They didn't say anything as they got to the nearest Floo point.
The fire didn't burn him.
But later, as sheets were destroyed, hands bruising hips, teeth dragging across skin, it was like being warm for the first time in years.
It was exactly like stepping into fire.