Darry's shows his love for his younger brother in several ways, he is generally considered the father figure of the family, and all the things he does to and with Ponyboy show his love for his "lil bro".
I do believe that Darry loves, Ponyboy and that he is only treating him the way he is out of personal interest. Darry just wants to keep Ponyboy safe because he is his older brother and that's his job.
