Mr. Whiskers thinks he is going to die. His last wish is to kiss a pretty girl. He never specifies Brandy, but Brandy thinks she needs to kiss him and keeps pushing it off. Then, moments before he is supposed to die, she kisses him.
Obviously, he wasn't dying. He wasn't actually poisoned and just believed he was because he was an idiot. Brandy then gets pissed off and chases him around.