In The Outsiders, why is Johnny the shy one?

Johnny is shy because he has a difficult home life and he was recently jumped by Socs.

Johnny is sensitive to begin with, but he becomes more sensitive after the Socs jump him.  This experience really traumatizes him.  The greasers can’t walk alone, or they get jumped by Socs.  Even though Johnny has a horrible home life, the beating from the Socs frightened him most of all.

"There was a whole bunch of them," Johnny...

Johnny is shy because he has a difficult home life and he was recently jumped by Socs.

Johnny is sensitive to begin with, but he becomes more sensitive after the Socs jump him.  This experience really traumatizes him.  The greasers can’t walk alone, or they get jumped by Socs.  Even though Johnny has a horrible home life, the beating from the Socs frightened him most of all.

"There was a whole bunch of them," Johnny went on… "A blue Mustang full... I got so scared..." He tried to swear, but suddenly started crying … I had seen Johnny take a whipping with a two-by-four from his old man and never let out a whimper. (Ch. 2) 

Johnny is unusual for a greaser.  He is quiet and deep.  Pony says that he is the gang’s pet, meaning that everyone loves and looks after Johnny.  Dally doesn’t hit him when he sticks up to him to protect Cherry, because Johnny is treated with kid gloves by everyone. 

Johnny’s experience at home is not as good.  His mother is distant, and his father is abusive.  Johnny feels like no one there cares about him. 

"I think I like it better when the old man's hittin' me." Johnny sighed. "At least then I know he knows who I am. I walk in that house, and nobody says anything. I walk out, and nobody says anything. I stay away all night, and nobody notices. At least you got Soda. I ain't got nobody." (Ch. 3) 

This is the reason Johnny refuses to even let his mother come in to see him when he is in the hospital.  As the doctors and nurses had figured out, the gang of greasers is Johnny’s real family.  He feels like all his mother does is blame him and yell at him, and all his father does is hit him.