In the end, it's all too weird. After everything Kenny has seen - heaven and hell, tits and balls, man, bear, pig, and everything in between - this is where he draws the line. Some of his friends are having kids. Ike moved away to college, and his sister is leaving come July. Everyone has started growing up, so why hasn't he?
He drinks in his truck and passes out in the back, alone under the the stars. He wakes up a couple hours later and starts driving, sobering up along the way.
This is the first time Kenny leaves South Park.