In the Outsiders, Johnny is the member of the gang that needs the most protection. He is only 16 and vulnerable. Johnny had been beaten up pretty badly by the Socs before the book even starts, and the Greasers felt like they had to band together to protect him. The Greasers were the only real family Johnny really had. He was abused by his parents, but within the gang, he had a place and he belonged.
We see the good side of Johnny when he puts his own life in danger to save the children. We see that his heart is full of love and compassion, he just needed the right place to show it. Pony comes to realize just how much Johnny needed them, and they needed Johnny. The Greasers needed a person to rally around and protect. It gave them purpose and made them forget, if for just a little while, how sad their lives were. All the members wanted was a family. They all came from broken homes and a rough past. With Johnny, they felt like they had a real mission to protect the young man. Johnny was weak and frail and young. They Greasers banded together with a common purpose of protecting Johnny.
Whey Johnny lays dying in the hospital, Pony reflects on how much they needed each other. It makes his death even sadder and harder to accept. The gang will grieve for the young friend that kept them all together. Now that Johnny has died, their purpose to stay within the gang is a little more confusing to them now. All Johnny had wanted was to feel like he belonged and that he was loved. Sadly he couldn't find that at home with his parents, but he did find it with the misfits that had become his family.
"If it hadn't been for the gang, Johnny would never have known what love and affection are."
At least in his short life, Johnny did know what love and affection are and what it feels like to be a part of something.
Johnny needed the gang for a sense of family, just as the others did. For many of the boys, the gang was the only real family they had because they had dysfunctional lives at home. There was a real sense of brotherhood within the gang. They could relate to each other and talk about things that they couldn't talk about with family members, if they even had family around. Also, Johnny was one of the quiet members of the gang. He was smaller and less aggressive than the others. He was seen as someone who needed to be protected in many ways.
The reason that Johnny needs the gang is so that he can have a sense of family. The other boys in the gang need that too. Without Johnny, that little brother character in the family is gone.