Energy
Tony has always been an spirited kid, running around, talking to everyone, asking endless questions. Never stopping. Adults say he will grow out of the phase, but he doesn't.
In Phillips Academy, he excels in all the classes he's interested in and stays up very late, making engines and primitive robots, hidden under a blanket with a torch between his teeth.
In MIT, he's the best student of his year, finding it easy to simultaneously keep partying all nights. He still has time to build his first functional A.I.s in the meantime.
When he takes over SI it's with a flourish, and he makes a revolution in weapon designing within months.
In Afghanistan, he creates the whole new self with his own hands, working day and night.
The reason to it all is neither redeeming himself nor avenging something; it has started much earlier. It's just – a child's idealism: knowledge and abilities mean you can change the world; it's a child's enthusiasm, too, and an adult's realization that there isn't enough time.
Tony's mind and willpower give energy to his body, not letting him rest, always urging him to keep going, to do more, and he gladly submits to them.