I don't recall aggressiveness being mentioned in the book.

Most of the people were just tenacious in a matter of fact sort of way. Not aggressive - just keep on walking out of the wilderness, as your clothes tear, your skin burns, bugs bore into your legs, and your shoes fall apart. Drink and eat things you find. Keep walking on blistered feet. (The lady who survived the plane crash really impressed me a lot hiking out in what was left of her dress without any real supplies.)

There have been many other stories I've read and heard that are similar - watch Touching the Void for a great example of just get-down-to-it dedication to the goal. Joe faltered, hallucinated, stumbled, passed out, but he just kept getting back up again, and again, to creep down the glacier one stiff broken leg drag at a time, dehydrated as all get out and suffering frostbite. Mental toughness maybe - not sure if it meets anyone's definition of that. A lot of the time he really wasn't in his right mind. But he accepted the situation right down to acknowledging that Simon and the other guy were very likely gone already, and he would be stranded and starving to death at the empty base camp - something in him just kept him stumbling along.

Not everyone could survive being left for dead in the ice at 15,000 feet with a broken leg - pretty much a miracle for Joe, despite his determination.