The answers to these questions can be found in Chapter 9. In that chapter, the greasers are getting ready for the rumble that they are going to have with the Socs. Pony is thinking about why various of the kids in the gang like to fight.
He decides that Steve fights out of hatred. He says that Darry does it because he has pride. He figures that Soda likes to fight just because it is fun for him. He then goes on to decide that here is no really good reason that he has for fighting.
Ponyboy says that Steve and Sodapop get into fights because they have too much energy. He says: "I can understand why Sodapop and Steve get into drag races and fights so much, though--- both of them have too much energy, too much feeling, with no way to blow it off." However, Steve is motivated to fight in part because he feels hatred inside, unlike Sodapop. Ponyboy says:
"I thought of Soda and Steve. What if one of them saw the other killed? Would that make them stop fighting? No, I thought, maybe it would make Soda stop, but not Steve. He'd go on hating and fighting."
In other words, Sodapop would not be motivated to keep fighting if he saw that it resulted in real injuries or death because he is not driven by hate but just an excess of energy for which he doesn't have any real outlets. However, Steve is motivated to fight out of hatred. Sodapop says about fighting: "It's action. It's a contest. Like a drag race or a dance or something." Steve, on the other hand, says, "I want to beat those Socs' heads in. When I get in a fight I want to stomp the other guy good. I like it, too." In other word, Soda likes the excitement of fighting, but Steve enjoys hurting people.
For Darry, fighting is another way to exercise his strength. Ponyboy says, "Darry liked anything that took strength, like weight lifting or playing football or roofing houses, even if he was proud of being smart too." Darry clearly enjoys fighting for the physical challenge of doing it, but Ponyboy, unlike his brothers, does not enjoy fighting at all and only fights so he doesn't disappoint his brothers and his friends.
The Outsiders traces the harsh reality for the "Greasers," a gang of boys who try to survive in an unforgiving environment where class distinctions are a good enough reason to fight and the rival gang, the "Socs", which is short for the "Socials, always seems to have the upper hand. The Socs are the "west-side rich kids" and the greasers are from the "East side." Greasers, Ponyboy who is the narrator explains, cannot "walk alone too much or they'll get jumped" by Socs for no reason.
Darry and Sodapop are Ponyboy's brothers and Pony describes Soda as "happy-go-lucky and grinning while Darry's hard and firm and hardly grins at all" (ch 1). Together the brothers help look after Ponyboy as their parents were killed in "an auto wreck." The Greasers have the occasional gang fight but Pony tries to stay out of trouble. However, Pony and Johnny find themselves changed forever after a series of events which include Johnny killing someone and the boys saving some children from a fire.
However, tensions are running high as Johnny has killed a Soc and the Socs have almost killed Johnny, he is so critically injured. Therefore, the Greasers talk about the "rumble" that will take place between the Socs and Greasers. Dally reminds them that "we gotta get even with the Socs. For Johnny" (ch 8). In chapter 9, the fight takes place. There are twenty-two Socs' and twenty Greasers.
Soda says that he likes fights because they are like drag-racing, "It's action. It's like a contest." Steve likes fights because he wants to "stomp the other guy good" and Soda suggests that Darry likes fighting because he gets to show off his muscles.