The gang had known Johnny to be too soft to ever hurt anyone, but he had a huge turning point when some socs decided to jump him and Ponyboy that night.
Bob had grabbed Pony and was drowning him in the water. Pony thought that it was going to be the end.
Johnny suddenly stabbed and killed Bob, and then the rest of the socs ran away.
This is an example of loyalty/love because Johnny would have never done something like that unless someone he was really loyal to was in danger. Johnny's reputation would have just let Pony die but he made a sudden choice to make sure Pony would be ok.
"You really killed him, huh, Johnny? (57) ... "They were drowning you, Pony. They might have killed you..."