The Colonel had off-handedly mentioned to him once that all girls had a smell. At the time, Ed had only just hit puberty and had no idea what the Colonel was talking about. When Ed was a bit older, he asked the Colonel again what he meant.
"All girls have a smell," came the same response. "Usually, it's perfume, or something of that kind of fragrance."
Ed asked what kind of fragrances. The Colonel had shrugged. "Strawberries, apples, flowers… sweet-smelling things, Fullmetal." Then the Colonel had given him that cocky smirk ( Ed hated that smirk ) and said, "Why are you asking me, Fullmetal? You're old enough. Go find out for yourself."
So Ed decided he would. When he met girls, he got close to them and tried to pick up their scent. Rose had smelled like the flower she was named after, while Bunny smelled of the outdoors.
He laughed sheepishly. "I'm probably going to break my arm again. So, if you could stay around and fix it when that happens…"
"Why are you trying to break my work?"
But Winry smelled of grease and machine parts, and Ed wouldn't have it any other way.