Not for Salvation
This is karma for teasing Luffy too much. Who knew controlling Devil Fruit power is this hard?!
Much like how Luffy's arms and sometimes cheek would just slide down the bed randomly in his sleep, Ace had to deal with his body randomly bursting into fire or just becoming fire every time he wanted to touch something. Like food. Burnt food was probably his biggest motivation in controlling his powers.
All four years have paid off and Ace no longer had any problems with his Devil Fruit. He could be completely human in whatever he does and whenever someone or something else is trying to make sudden contact with his body his power automatically made his body untouchable.
Now, he needed to go against that instinct. Trained for four years, for this one moment of standing between his baby brother and the Red Dog.