Why is the gang so upbeat and excited while heading to the rumble? in the outsiders/?

The Rumble represents what is perceived to be a necessary physical release for this group of young males. In addition, the gangs agree to not bring weapons, so the perception is that it is unlikely anyone would be killed. In essence, a physical thrill that releases Endorphin and reassures the victors of their masculinity with a need to "dominate" others physically.