"It is possible you will continue growing, but more likely that you will lose what control you have managed to maintain."

As strong as Steve has been to this point, he is still mortal and all mortals have their limits. Without help to bind that control so that it would only unleash on rare occasions, he would be unable to do it for himself.

"And this does not concern you any longer, this is not a mortal matter."

Bucky stepped forward at that point, brow furrowed. "It's our concern. If you're taking Steve someplace dangerous, someplace that people might say he doesn't get to live, then he doesn't go without someone to watch his back."