1) Because they got mad for them hanging out with Cherri
2)Because Dally wanted Johnny to become independent and fight for himself. He was proud he showed his anger to the Socs, and so he would be less sensitive and weak.
3) It symbolizes how it's a new change/beginning for them. Similar to the sunset which Johnny and PB watched, it says nothing gold can stay because they seemed like they were stuck depending on the other members when suddenly they must fight for themself.
