Why was Johnny's death so hard on Dally?

Johnny was like a little brother to Dally. Dally's world falls apart after Johnny's death. Dally wants to get into a shootout woth police: he wants to die. It is Ironic when Dally tells Ponyboy, "You'd better wise up, Pony . . . you get tough like me and you don't get hurt. You look out for yourself and nothing can touch you."