Winry thinks it started with the Elric brothers. Those boys who loved to "deconstruct and recreate"—a sophisticated way of saying that they broke things down and rebuilt them. In mechanical terms, they determined what made something tick and then crafted it into their designs piece by piece.
She wasn't interested in chemistry and circles but had wanted to understand why alchemy held such an appeal for Ed and Al. She grasped the basic concepts: determining composition on an elemental level; breaking it down with energy that flowed through a circle, reminiscent of magic; restructuring the elements until they form something else.
She'd been fascinated by the creativity aspect of alchemy as a sort of artistic science. Still, it was not for her. She could not progress past the law that output equaled input ("Equivalent exchange!", Edward had reminded her) and make sense of it. The brothers had studied and immersed themselves deeper in their father's science while Winry had been left with a longing to understand how to build wonderful things the way they could.
Studying something closer to home became a fix. Her parents' medical books were as easy for Winry to comprehend as Hohenheim's alchemy texts to the Elric boys. Anatomy, how the body functioned, how to treat illness—all of these changed lives. She dreamed of making the greatest innovations in medical science as a prominent doctor.
These fantasies stayed with her through the death of her mother and father. She could honor them, make them proud, carry on their work and advance it. But beyond wistful moments of recollection, her future stayed beneath conscious thought. Getting a degree could wait ten years.
Winry thinks her mind changed when the boys' lives did, when Ed lost his limbs and needed to walk again because Al's entire body could not be replaced with automail. Winry had toyed with the idea of becoming a mechanic too, but her heart had been in becoming a doctor until she saw the miserable expression Ed wore when she'd feed him, bathe him, and help him do other menial tasks he could no longer do on his own.
When Mr. Mustang had arrived and put a spark back into Edward's eyes, Ed had put a spark into Winry. She'd poured over long-forgotten designs and started working with Granny. She had built Den's automail—a simple piece because a dog needed less complicated movements and they didn't connect it to any of the nerves—so Winry was certain she could build something for Ed.
The subtle joy on Ed's face when he could walk his own turned Winry's spark into a passion. Her medical texts seemed dull and grim compared to the gleam of the metal and the beam of the automail patients she saw. Treating sickness and watching people die couldn't compare to watching someone's life rebuilt with a few bolts, nuts, and steel.
Winry often muses on how those brothers shook the foundations of her world and rebuilt her into something slightly different. On the path to becoming a doctor, Winry had become a mechanic instead. It had been for Edward's sake at first, and then for the sake of the patients she could help, and finally for herself.
She could find fulfillment in reconstructing shattered lives, not with the same flesh and bone, but with something different. Something that couldn't restore what they had lost, but could replace it.
The pieces of Winry's life are so far away. A leader she's never seen, a pair of boys she never sees anymore, and an enigmatic science have had their parts to play. Her parents. Her best friends running into danger.
And all of those pieces have given Winry her passion. They've made her into who she is. She sketches designs for Ed, replacing what alchemy hasn't given him back yet. She works late into the night for another country boy injured by a saw blade. She scolds herself for not making a piece better than last time for the legless soldier whose life was changed unapologetically by the same military that changed hers.
Alchemy. The Elrics. Her parents and her Granny. Amestris and its wars. They've taken from Winry and they've given, and now she is giving back in the only instance of one being all and all being one that she can understand.
It's equivalent exchange too. Winry hopes that if she gives back enough, if she works enough all-nighters and makes her metal strong enough to keep her friends alive, those pieces of her life will come back whole.
Until then, and after, Winry studies, understands, and reconstructs, hoping she can repair a part of the world with the pieces she has at hand.