The Cold War - S1-E8
Continuity mistake: Kel drinks a fair bit of Kenan's "medicine" but in the next shot the amount of "medicine" is greater than before.
The Cold War - S1-E8
Continuity mistake: The clock in Rigby's is changing time throughout. (00:04:00)
Answer: He's simply enjoying doing it himself. Especially since he likely believes he can do it better than she can.