Do you think Darry loves Ponyboy in The Outsiders? Why does he treat Ponyboy the way he does?

Darry does love Ponyboy, but he feels a lot of pressure to be both brother and father.

Darry and Pony’s parents died in a car accident, leaving Darry as the parent because he was the oldest brother.  Pony says that Darry treats him harshly compared to his older brother Soda, but this is because Darry feels under pressure to make sure Pony turns out right.

I love Soda more than I've ever loved anyone, even Mom and Dad. He's always happy-go-lucky and grinning, while Darry's hard and firm and rarely grins at all. But then, Darry's gone through a lot in his twenty years, grown up too fast. (Ch. 1)

Pony comes home late one night and gets in a fight with Darry.  When Darry hits Pony, Pony reacts by running away.  He feels as if Darry doesn’t understand him and is too hard on him.  He runs because he feels that Darry has treated him unfairly.  This does not mean that Darry does not care about him, though.  Darry overreacted by hitting him because he cared too much. 

After Pony returns from the incident with the burned church, Darry seems to make more of an effort to treat Pony gently.  When he got mad at him and hit him, Pony ran away and ended up getting involved in the killing of a Soc.  Darry realizes that Pony has been through a lot and wants to help him.

The incident where Pony decides that Darry really loves love him starts with Darry scolding him for having a cigarette in bed.

"Maybe you can be a little neater, huh, little buddy?"

He'd never called me that before. Soda was the only one he ever called "little buddy."

"Sure," I said, "I'll be more careful." (Ch. 11)

Pony appreciates the pet name, perhaps even more than Darry realizes.  After this, he feels that the family will be all right.  He knows that Darry does care about him, was worried about him, and is just looking out for him.  From this point on, Pony and Darry have a stronger relationship.