Why is Darry so hard and tough instead of like Soda?

In The Outsiders, Ponyboy has two older brothers with personalities that are quite different from each other. Soda, Ponyboy's second oldest brother, is described as being "happy-go-lucky and grinning." Soda gets a substantial amount of attention from girls as he carries out his duties as a mechanic at a gas station. As the middle brother, Soda often serves as a buffer between Ponyboy and Darry.
Darry, the oldest brother, is seen by Ponyboy as "hard and firm." Ponyboy says that Darry "doesn't understand anything that is not plain hard fact." Since their parents died in a car accident, Darry is the one in charge. The boys can remain together only if they manage to stay out of trouble. It is for this reason that Darry seems so much tougher and harder than Soda. Although he is only twenty, Darry works "like an old man" to help support his brothers. Darry was successful and athletic in high school, but cannot afford to attend college. He hopes that Ponyboy will have opportunities that are not available to him, and therefore pushes Ponyboy to achieve.